New Grandpa Bear/ & also Fisher insert Owner. Looking for advice from Coaly.

Welcome to the Forum;

The Insert was designed before liners existed and was UL approved when installed as shown in the manual. With the use of a "boot" to connect to a liner it will connect the same as any new insert. The main factor is not installing into a hearth with a huge flue that was the norm for a fireplace. You simply don't allow enough heat up a larger area than the stove outlet to keep it clean. A few advantages are not removing the Insert every time you sweep the chimney, and all the air will go through the Insert instead of around it if there are any leaks at the face plate seal. Any air that leaks around the faceplate rushes up the chimney cooling it, and prevents the needed oxygen from going through the Insert. So there are definite advantages. Local codes are going to vary as per installation, but normally when an appliance was installed up to code at the time, and was UL approved, as long as it remains unchanged, it is still approved. UL listings don't expire as far as I know. The problem with the installation is that it can become unsafe if the faceplate leaks or is used incorrectly. By all means, every one of these Inserts should be connected to a liner with what we know today about the history of problems with the installation.

It is a Grandpa. I wouldn't invest in an 8 inch liner since most any newer stove is going to require 6 inch. Codes do not allow the reduction of pipe or flue smaller than the stove outlet. The problem is, that code was written to prevent anyone from choking off a stove since the outlet was normally sized to the square inch of the firebox. The firebox size of the Papa Bear is just as large as your Grandpa and was built with a 6 inch flue. So If you want a Fisher, That's the stove I'd recommend connected to a 6 inch liner that could be used with a newer stove in the future. You lose the ability to view the fire, but gain a whole lot more.

That said, your stove will physically work fine connected to a 6 inch liner. The reason for the larger 8 inch outlet is due to open door burning with spark screen in place. Many are in use reduced down as well as Inserts reduced to 6. A quick search of Craigslist will show many for sale with 8 to 6 reducers, and a few members here have added baffles with both stoves and Inserts reduced with no problems. Not saying it's legal to do so, but they physically work with no smoke roll in problems with a good chimney. Like an engine with a restricted exhaust or intake, you're limiting output and won't get full throttle, but that much heat is rarely needed. Another plus is the faster velocity in a smaller flue decreases dwell time. The larger the flue, the slower the movement and the longer the smoke particles have to cool and stick.

I've got a new desktop for Christmas ! Thanks !!

Every year my wife makes me a calendar with a stove picture each month. I'll bet yours makes the cut for next December.

Sometimes what you need and what you want are two different things. Bob was a blunt kind of guy and would sell you what you NEED, not always what you want, so you would be satisfied with the efficiency in the end. Unknown factors like if you have an unlimited wood supply and square footage are main factors to consider.

You didn't mention the flue size above the Insert now, so that would be a major factor in the consideration of use as is. (also if they cleaned the entire flue and removed Insert to clean behind it) The reason for rapid creosote formation installed without a liner is having a much larger flue built for a fireplace.

How many square feet are you heating with the Insert? It probably should have had a blower or two as well. (They were available single under the bottom or dual on the sides. A new unit is not required to add a blower. ( Here's a good thread for homemade or original blowers; https://www.hearth.com/talk/threads/made-a-blower-for-my-fisher-insert.117672/ ) The portion that sticks out into the room is for radiant heat as well as the cast doors. It gives you a place for water kettle for humidification and cooking. With a power outage they continue to heat with natural convection around the box in the back like yours does now, and radiate from the portion outside the hearth. You loose that with a more flush unit that requires a blower to convect more than radiate.
These are the things you need to cook and heat with no electric. That is what the Insert was designed for, that may not be what you want.

The later Grandpa had a Smoke Shelf Baffle and I would certainly install one.

You'll find running a stove hard with lots of draft raises the flue temp more than the stove temperature. It's finding the balance between stove temperature and flue temperature to get the best efficiency while keeping the chimney clean. Like cruise on a vehicle, that isn't wide open throttle. Open door burning with screen in place is as close to "full throttle" as you get. ALL your heat is going up the chimney and you're giving it as much air as possible. It isn't even considered a radiant heater in fireplace mode. A bigger flue isn't better with a stove. A bigger flue takes more fuel to run the stove.

The liner gurus can give you an idea changing sizes and lining liners with smaller sizes. It's only a cost factor.

I assume the brick wall next to Grandpa is solid non combustible and not in contact with any combustible material?

I believe the newer stoves that require 6 inch must be 6, not 7. They will not work. The low pressure created in the chimney is what causes the air to enter in their tubes that look like a gas burner in a BBQ grill. Think of it as sucking up a tiny cocktail straw compared to a big soda straw, or garden hose. It takes much less suction to pull air through the smaller straw. That is done by atmospheric pressure with very minute pressure differentials. I'm sure these guys see the double doors reduced down to 6 all the time. Most people have no clue that you buy a stove to match your chimney size, or need to make the chimney the stove size.

If you take a piece of cardboard and cut to the size you need, when set it in place, you have a template for the steel. Some fabricator shops have steel plate in stock to cut it for you, others order it pre-cut to the size you need from their supplier. There may be a small cut charge, but it's not expensive. You want "5/16 HR steel plate A36". That is Hot Rolled mild steel, 5/16 thick. I pay 1.42 / lb. sheared to the size I need. They are usually 10 to 15 lbs. around $20.

Welcome to the Forum !
7 inch works fine and you won't have any smoke roll in unless the larger chimney flue isn't warm. That would happen even with 8 inch all the way. Yes, code doesn't allow reducing. The size of the firebox is no larger than a Papa Bear with 6 inch outlet, but since yours was made for fire viewing with doors open and a screen in place, the outlet was made larger. Even people who have added baffles with reduced flues don't have smoke roll in problems with a good chimney. (Maybe yours is the more common 7 X 7) ?
If there is any single wall at the adapter closer than the 18 inch clearance, simply make a heat shield above it with at least 24 gauge sheet metal or piece of single wall pipe with at least 1 inch clearance between pipe or ceiling. There are commercially made ones as well as seen near the bottom of this page;
http://www.rockfordchimneysupply.com/singlewallblack.php Just like with double wall pipe, the clearance is reduced to 6. Some people just don't like the looks of a shield, but they don't have to look like a piece of tin from 1880 above the pipe ..........

You could have saved the expense of the double wall pipe, and possibly over crimp the 8 inch end into the thimble with hand crimpers? They shrink the crimped end down much smaller than a factory crimp.

The risk of chimney fire is greater with a flue larger than the outlet on the stove. The larger the inside diameter, the more heat required to keep it hot enough to draft properly and not condense inside causing fast creosote build up. Plus, the larger the flue, the slower the rising particles, increasing the time they are in the flue allowing time to cool and condense sticking to the sides.

Thanks for the update!
Let us know if you have questions with the baffle plate. Pick up two firebricks (1 1/4'" thick) at a masonry supply store if you have any locally. They are only $1.50 to $3.00 each, depending. Then when the stove is out, make a cardboard template the width of inside across stove and from back above existing bricks aim it towards the bottom bend in top. The open square inch space (smoke space for exhaust to travel) can be 38 1/2 square inches in yours with that chimney. Set the bricks on edge above the existing ones, and set the cardboard on top to measure the smoke space opening. Slide bricks fore and aft to adjust tilt towards the bend. When you get the template the right size, you will have a pattern to cut the baffle plate to match. Or just order a piece of 5/16 thick A-36 steel plate from a steel supplier or fabrication shop. They order steel all the time and supply companies shear it to the size you want for a small cut charge. Should be no more than 30 to 35.00 and ready to set in place.

Here's a video in post #28 with a stove not only reduced to 6 inch, but a baffle added for his chimney size as well. This puts to rest how well they work reduced to 6.
https://www.hearth.com/talk/threads/cant-seem-to-figure-out.119184/page-2#post-1617042

Silver anti-seize works good and is higher heat resistant than most greases. Carefully tap the pins up and out if they move easy enough. May need penetrant and rotate them to remove. Wire wheel or remove rust and coat with lubricant. They should swing free and easy and the pins should lift right out.

Most call them Draft Caps, they were patented by Bob Fisher's dad Baxter as Air Dampers.
Reach in from the back through the air intake hole and brush silver anti-seize or high temp grease on the threads.

Keep both of these moving parts greased since they are the only wear items on the stove and worn hinge pins and holes can let air leak when the doors become misaligned. Some Draft Caps have steel inserts that is actually a steel nut cast into the aluminum cap, so they don't wear as much as the cheaper all aluminum caps with softer threads. They should spin free without much wobble.

You want the first manual Pre '78 that includes Papa, Mama , Baby as well as the two double door stoves Grandma and Grandpa. It's the only manual that includes all the models at the time. That is NOT a UL listed manual and the stoves didn't come with one until customers were asking for one. The Pennsylvania fabricator came up with it and they were supplied with stoves until Fisher printed their own for the UL listed stoves.