Has anyone run 295/50/15 on a 10" rim with 5" back space on a E-Body without any problems using stock springs?

I don't think 5" backspacing will be enough.

To center a 10" rim between the wheel wells on my challenger, with stock width axle is 6.5" backspacing . This spacing will interfere with the stock 46" leaf spring spacing.

5" backspacing will clear the stock location leaf springs eaisly, but put the outside edge of the rim into the fender lip.

Get a Dr. Diff 1" relocation kit.

w/a stock e-body rear end, you need 6" BS. W/a '66-'70 b-body rear end, 5" BS.

we have 15x8 wheels with 5 inch of back spacing tires are 275x60,sx15 and we had to run a 1/4 spacer because both tires rubbed on the springs(all bone stock)
you will not get a 295 50 under the car unless you move the springs in some way or you raise the car and let it stick out some bit

15X8s with 4,5" backspacing. 295-50-15s.