16x8 = yes so far

Is that a stock 8 3/4 A-body rear end? Converted to LBP (Large Bolt Pattern)?

Or another rear end.

Stock 8 3/4 with stock axles should take max 4.5" backspacing with 8" wide rim and 225/50 tire. If LBP axles, then 4.25" backspace. You could run 245/50 w/4.5 backspace and might need fender lipping. But 70 Duster lip might not need to.

Think you went backward with the stock vs LBP axles there. With stock, or SBP axles the maximum backspace for a 15/16" rim will be about 4.5", give or take depending on the rim/tire options. With the LBP axles and their wider flange offset you can get closer to 4.75" or even 5".

On my '71 Dart with an 8 3/4, LBP axles, and stock spring locations I'm at 6" from WMS to spring, so, if you consider about a 1/2" of clearance and roughly a 1/2" of tire overhang you're at a 5" backspace. That should be the same even on an early A since the axle width and spring locations are the same. Obviously with wider tires you may need a bit less than that, as you may end up with more tire overhanging the rim than that.