Where are you wanting to put the futon? In the front curved end right of the door or on the side beneath the windows across from the galley?
In our '64 GT, we have the gaucho across from the galley and it makes into a 54" wide (wider than the original 48") bed leaving about 8 or 9" to pass in front of the galley. You really can't open the fridge much though when the bed is down, maybe enough to squeeze out the cream for coffee in the morning.
The front area which is a dinette in ours, makes into a bed about 36" wide without impeding on the door. A futon could block a portion of the door when opened.
The trailer is about 7'-6" wide (side-to-side) most futons are not that long so you should be fine either way. I guess part of your answer lies in the design of your futon and where you intend to use it. I would think a "full" would work better than a "queen" mattress. A "King" is probably out of the question in a GT.