17x8 wheels on a 68 Cuda Fastback

What Steve said. All depends on the tire you want to run. But, with those 4.75" backspace rims you probably won't even be able to run 245's without cutting and rolling, even on a Barracuda. I'd be sending them back. The 17x8's with 5.5" of backspace won't hit the tie rod ends or UCA's either, and you could run 255's on those.