I'm running that same DrDiff set-up on my '70 with 325/50-15's. I did clearance the fender lip slightly just in case. I also used steel 1/4" spacers that I double checked fitment on the rim's mounting flange to fine tune the tire location within the wheel well. I also had to grind the leaf spring on the side slightly to add more clearance. They don't rub anywhere.

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1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)