I have 295-50's on 15x8 SS wheels (with 4.5" backspacing) on my '70. They were pretty close to the outside fender lip, so I rolled up the inside edge of the aluminum wheelwell trim just to be sure it didn't hit (not the sheetmetal, just the trim). They have no clearance problems and the tire size looks great on the back. I say go for it!
I am running 15x8 ralleyes with 255 60r15 tires on my 71, and I have a good amount of clearance with the wheelwells, I'd say 4 inches. However I have the cargo coils with some non-functioning shocks, so I could scrape tires on corners or big bumps.
I think you can fit those in there, but you might experience tire rubbing problems at times.
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