I agree with Cale, you will absolutely need a real drag slick to be able to get the most out of it.
When I had a stock widht rear axle in my mustang with original bb drums, I was using 15x10 wheels with 6" of backspace, that was just perfect match, I had 325/50-15 ET streets and it did fit ok - I had standard eye leafsprings so I got the space between tire and fender. Once the rear was lowered, thent the tires came too close to fenderwell.
Here's the picture with original rear axle and with the standard height leafs and 15x10 6" bs and 325/50-15.
15x10 with 6" bs you should be just fine with 295/50-15 tire, no rubbing at all. I still would go as tall tire as possible 28-29 1/2" as usuall I do, but in your case, you have 4.11 gears and you got the torque of a bigblock. So tall tire will downsize your gears so "use the Force"
You will need to measure the correct widht, bs etc. If you have a Cobra rear brakes, it can affect 1/4" for the rear width per side.
I ordered a new M9 rear from Moser and it was shortened 1/2" per side from the original '71-'73 rear widht only because I wanted to be to use Weld Prostars with 5,5" backspace. I am running 15x9 prostars with # 3056ST 29.5 x 9 - 15 ET Drag slicks and the compound or the sidewells are made for manual transmission cars. Fitment is as good as it gets - I got lucky obviously!!! Section is allmost 13" wide and the threat is 9" as manufacturer announces. Here's a picture from my current combo before the fenders were rolled: