We used to make top vent babys all the time. They work well like any other TV fisher. Coaly may have some specs he can provide on this. We always told people to intall a damper since the Tv was not as efficent. If you put the baffle that Coaly installs it should help also. I have never done one.
Just some help if wanted:
1. measure in from the top to the back of the stove. Add 1/2" to this measurement. Mark the new hole with soap stone or something that will stay while torching. I always used the pipe and centered it left to right and double check the dimension from the back. tip I always started at the back ,angle the torch to the front. It is to be sure to clear the acutual back of the stove. I always left the line when traceing the pipe, if not it will be to big of a hole.
2. After tourching the hole check the fit. I always prefered to have to drive the pipe in with a shop hammer than it to just fall through. It made easier welding.
3. We used 1 1/2" angle scrap to use as a spacer to judge the height of the new pipe. putting it on the stove top and tack the pipe to hold in place.
4. weld it up.
note:
1. use 1/4" plate to cover the hole on the back after cutting out the old pipe.
2. There are all kinds of 6" pipe. be sure it is 6"od. with out seams. 1/8" thick. a 4"-6" length is adequate. If you put the wrong pipe in it will be a real headach to attach your black stove pipe. These required a double skirted adapter when hooking up your stove.
3. If you can find 6" Id pipe with a 1/4" wall you will not need the adapter. A short piece of this may be harder to find but worth the effort. These were used on the later model stoves and a 2" length is all you need. this would be put on the stove top, centered and weld the inside of the pipe to the stove top. Then torch it to provide the exhaust hole.
Good luck, I always enjoyed the smell of cutting and welding on a burned stove. lol I must be sick
If something here is not clear let me know, I will talk you through it if you need it.