I never tried to fit "big" tires on my '69 RR, but all the fitment comments above seem to mirror what I've found w/ my '73 Chally (standard location springs and stock ride height).

8" w/ 4.5" backspace works with a tire up to about 11.5" sidewall; a typical 275/60 is about 11", and 29.5" x 9" slick is about the same, a 295/60(?) was 12" and too tight a fit w/o messing w/ the inner fender lips

10" w/ 5.5" backspace and 275/60 is also about 12" sidewall, but the shorter tire fits a little better... still modded the inner fender lips for a little more room at the top


2021 Challenger 6.4L Scat Pack 1320
100% stock: 1.680, 11.894 at 113.75 (DA 175 ft)
weight reduction, wheels, tires, Hellcat air box: 1.661, 11.686 at 115.97 (DA 710 ft)

1973 Challenger 452 ci street/strip [2008]
pump gas, DOT radials: 1.454, 10.523 at 126.44 (DA 514 ft)