help with boiler sizing

I have a gw 100 need help with sizing a unit i am in discussion with greenwood about the size of my boiler in an earlier post someone asked if i was undersized well not a good question to most men but guys i need help.
Heres what i have a 1750 sq. ft. poorly insulated home built about 1920 plaster and lath walls lots of single pane windows etc. 100,000 btu heat exchanger in a forced air furnace.
about 800 sq. ft. of basement with 100,000 btu heat exchanger to heat the basement
350 gal hot tub
clothes dryer used a small 6x6 heat exchanger
talked with a central boiler dealer from rio grand ohio said i needed a 500,000 btu furnace
greenwood only makes a 300,000 btu furnace
by this time i no longer trust my installer he is a very nice guy but...... not real knowlegabble about greenwood
a gasification system was important to me secondary to i live in the city with lots of trees around ( like living in the woods)
also heat hot water with a sidearm at the present but will probably go with a plate heat exchanger. the distributer said they would probably exchange for a plate exchanger. ( what size should i use from 40 gal electric tank with three people on 14 yr long showers. )
i really appreciate all the help in advance.

i can't keep my temps up when heating both hot tub and the house especially when it get below 20 degrees.
If i only heat the house and dhw i can keep temps up to around 170 deg range.
When i try to heat both i come home after about 9-10 hrs. to pipes that are 80-90 degrees. takes about 2-3 hrs to get back up to temp.
thanks doc

Well i need all the help i can get.