Figure out what tires you want to run first. Then figure out what size rims you'll need to do that, and what backspacing they're available with. Most newer, commonly available wheels have a lot more backspacing than the classic wheels. You may not want to narrow the rear at all. I have a 68-70 B body 8 3/4 in my Duster. It's a full 2.3" wider than the original A body 8 3/4", and it works perfectly with the 18x10's and 295/35/18's I have on it.
What you don't want to do is arbitrarily narrow the rear axle, then find out you need to pay for custom rims because nothing off the shelf will fit.