Check around to make sure, but I think the rear end is going to have be narrowed to make it work (to keep the tire inside the well, they will have to move inward, thus the narrowed rear end).
Kinda like the chicken or the egg as far as wheels (offset/backspacing) and the width of the rear end. Either pick your wheels and have the rear end narrowed to fit those or narrow the rear end and pick the wheels with the proper offset and back spacing.
I think it is easier to pick the rim and then narrow to the proper width. My rims (I bought the ones off the car pictured) have a -11 offset for a bigger lip and 5.5 back spacing. I need to mock up the tires and measure, then I will cut down my 9".
I know of at least one guy on another site that has a '65 fastback that has been tubbed. Might be worth sending them a pm to see how far you can go without modifying the backseat. Let me know.
1968 Fastback: 331 stroker, FTI cam, N351 Heads with a few upgrades, 4r70w w/3600 stall, 9" locker w/3.70 gears