63-66 Dart. How wide of a tire on the back?

The rim width and offset are important but the last measurement in the equation is the tire size. Here's a refresher of what I run:

I put 225/70-15's on my 66 on 7" wide wheels with 4" backspace. About a 1" clearance both inside and out. I think you could use a 8" wheel with the right offset and just the right tire without rubbing. Most have to trim or roll the fender lip to prevent cutting a tire. tmm
It's what ever works. Here's a handy chart: http://www.team3wheels.com/tech_specs_tire_chart.html