Does anyone know what would be the absolute widest 15 inch rim and tire I can use on the back of an 85 SS without having to modify anything? I remember back in the day when it was cool to have the back tire really wide and part of it sticking out...and I still want it as wide as possible but I'd rather keep it tucked and lower the elky slightly.

Click the search button, type in tires, search with suspension. This question has been asked numerous times. :)

Search is your friend. Your best bet is 15x8.

You might try a 15X10 with a 5" backspacing. I'm running 17x11 with a 5" backspacing, and they fit with a little fender lip work. I would try a P275/50R15 or P295/50R15 tire.