G'Day TAC.

You say 315's but do you have a specific size in mind? Being that you mentioned Forgelines, I doubt you are talking 315/60-15's, which are different that 315/40-18's.

The dished look you are after will likely require a narrowed rear, but you do not need a narrowed rear to simply run a wider tire.

the 66-67 B bodies have a restrictive wheel opening, you are only able to fit a ~28" tall tire without stretching, trimming, hacking the wheel opening. Relocating the springs and tubbing are necessary for the width.

On my 66, Springs are relocated, fabricated tubs, 70 B-body width rear

Pics in order are:
315/60-15 on 15x10 w/ ~7.625 BS
325/50-15 on 15x10 w/ 7.5 BS
295/55-15 on 15x10 w/ 7.5 BS
same