biggest tire size for A-body plymouth/valiant

I've had 255/60/R15's on 15X7 rims w/4 1/4 back space on my 64 Dart. Stock width rear w/drum brakes. Just an fyi when going to rear discs they add 1/4" to the width... It's already tight in there... I now have 17X7 4.25 bs with 235/50/17's and no issues.

Bob
64 Dart 270 2dr sdn
360/904/391
Hello BOBSDART, Have a 1966 valiant and would like to also put 255/60/15 tires on my car but the rims I am looking at are 15 x 8 / 4.50 backing and 0 mm offset. Would these work ? I have 15x8 /3.75 backing and -18.80 /-19 .20 mm offset and they will not work stick out past fender well . I would appreciate your advice very much . Thank you Richie