Well 15x8 with a 4.25” backspace would be a bad plan for BBP axles and an 8 3/4 if you want to run the biggest tire possible. SBP would be a different story. Unless you’re running SS springs and plan on the tires hanging outside the quarters.
And he asked for the “biggest”, not the safest bet. But literally dozens of people just on this board run 275’s on Dusters/Demons/Dart Sports. And most of them running BBP axles run 15x8’s with a 4.5” backspace too.
But we’ve had this conversation before. If he doesn’t want to take my word for it, he can check a handful of other members Demons/Dusters/Dart Sports that I summarized here, all with 275’s.