Want to know widest 15 " tire and wheel size and backspacing to fill rear wheel well without rubbing

Need help with 74 Dart Swinger rear wheels with new 440 RB transplant.
Currently stock 14" rims. I need a 15" rim with some backspace.
I do not know a lot about those, The rear wheel wells are stock, no tubs.

Then I will need a tire size for the rims. This Dart has the C body 4.5 spacing 14" rims.
I might fit a 60 series wheel onto the 14" rim. But I think that would defeat the purpose of hooking up with 390 HP.
some CLUB member has done this already. I would be grateful to have the correct sizes.
Thanks,
DDume in VA