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!