I like 235/60/15 on 7" rims front and 255/60/15 on 7" rims rear. If you go with 8" wide rear rims then the 275/60/15 is a nice look.

70 Road Runner 383-4 4 speed FJ5 & black guts
70 Charger R/T 440-4 4 speed FJ5 & white guts