My only concern would be the depth of the futon in the "couch" position...the space in front of the door to the shell is pretty limited and I don't think you would want the arms or seat sticking out into the doorway all the time. If you can get so "out-to-out" dimensions before purchase that would help. Our GT which is a couple of years older and 1' shorter than yours has 36" in front of the door. I'm not certain what a 20' 1966 has...but I don't think it too much more maybe 4-6".
Comfort? That's kinda relative. I have a futon in one of my guest rooms at home...it's pretty comfy, but I know there are different qualities/construction of the materesses available. The key is in the mattress - lay on it to test them out in the store to see what feels better to you. Ours was the best available at the time...it's about 12 years old now, not used very much but it's still comfy. They do tend to get a crease at the fold from being in the seated position more than the bed position - but, that's to be expected IMO.
Good luck in deciding what's best for you ~ post pictures when you can.
Shari
