69 Dart Wheel offset

I am moving my leef springs inward using a spring relo kit. the new bracket will be attached to the frame rail so i can fit a wider tire on rear. Does anyone know what the offset for the wheel needs to be to maximize the space? I havent decided on the wheel size yet, thinking maybe 15" steel wheels or 19" aluminum...... Also what is the widest tire that will fit? I was thinking 13"

Thanks in advance!

So you moved the springs inboard 3" correct? Is this car mini tubbed or does it have factory tubs? If factory tubs, 275's are about as far as it goes I believe. With mini tubs, around315/335's. I assume you want to know the offset for the wheels with a factory width rear?