Dart & Valiant owners help w/ tire sizes

I'm needing to know if a 255 tire will fit in the stock 67-69 Valiant wheel opening without a leaf offset kit, or using SS springs, etc. I'm possibly game for a little lip trimming, but that's it.

What backspacing? From research, it's looking like 4.75 to 5.00" BS is right on a 8" rim

Or are we better off with a 7" rim? I'm trying to fit a 255 in there. :coffee2: