I tried 315/50R15 on my 70 RR but they rubbed. Went back to 275/60R15 and they clear ok. Wheels are 10" with 5 1/2" backspace and the tyres are centred in the wheel well.

1970 Road Runner 505 cid MCH CNC ported Stealth heads MP 528 camshaft 4 speed GV overdrive 11.98 @ 117 on street treads