As old as this posting
is, I would imagine that you have already made the filler cushion and supports for it. Put I will add my 2 cents worth, even if too late.
I too am leaning heavily towards acquiring a mid 80's Cat 36. I have been reading eveything I can find on them. Seems that boats made prior to 1985 did not come stock with that filler. 1985 and newer did. I agree with Mike in LA, I doubt it would be difficutl at all. I also think that the space would make a nice extra storage
area. If I purchase
prior to 1985, I think I would make the filler cushion two pieces so it would be easy access to storage
under and aft and if you wanted to leave room to crawl out of berth by only using aft portion of filler. Two half pieces probably easier to handle and store if not in use also. I always though that would be the berth I slept in, so for me I doubt once I put the filler in and put in bedding, that's the way it would always be.