Rears are going to be real close. My understanding is that 275 is the biggest for the rear without having perfect wheel offset and a well centered rear end. Going bigger may require moving the springs inboard, which isn't hard. There is a sticky over on E-bodies dot org that has a lot of discussion and pictures. Probably had that discussion here before but it's not stickied. I do know that at least a 315 will go on the front and rear with nothing more than rolled fender lips, if you get the rear springs out of the way.
There are lots of onlinebackspace calculators that help you figure out what will fit based off what you currently have.