Recommended rear tire size for a stepside?

[Lil Ape]

I have an 83' stepside that's been lowered a little by the previous owner. No flip kit in the rear for right now. And am running factory 235/75/15 on 15x8 wheels currently. Im looking to go wider in the rear and was thinking 275/60/15. Looking to run drag radials mostly for the look, as the truck doesn't get driven often. I like the nitto NT555r's and heard good things about them. The widest they offer is a 325/50/15 which i'm not sure if they will be too wide. Any recommendations on what might work?

And I also plan to swap wheels with what ever tire size i go with. Would 15x8's be a good wheel width for a 275 wide tire? Or if i go larger than that, should I go to a 15x10? Recommended backspacing? I should probably get the backspacing on my current wheels, that may give me a better idea what to run with.