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.
