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