What Wheel Backspacing for an F-Body 8.25 in an A-Body?

Trying to line things up for my planned 8.25 swap into my 68 Valiant. I have wheels that are 15x8, 4.5"bsp, bbp, and just mocked them up on the rear outside of the car, and it measured about 68" from the edge of the wheel to the edge of the wheel. I'm not feeling great about that width, especially with my planned 255/60/15 tires (I'm willing to do 235s, but I'm trying to get a taller tire to bring down highway rpm with a more aggressive rear end ratio). I can't find a wheel in my price range (cheap) with the bolt pattern and more backspacing than 4.5. So are the 255 tires just a dream and won't fit? and will the wheels I have now clear the fenders with any tire?