I want to put on 15" Rally wheels. I see people run a 15 x 8 rim but don't know what back space or tire size I can run to have the biggest tire possible.

I run a 15x8 wheel vintique rally with a 5" backspace and a 275x60x15 BF Goodrich tire. It's tight (about 1/2" inside and out) but I didn't have to do any triming and don't have any rubbing issue's. Check the pic in sig to see what they look like.

I'm running 15x7 with 4.75 backspace tires are 235x60x15

Left front rubs sometimes on a un-even tight turn. Front end has tall spindles with 1" shorter springs.

No problen with the rear tires. I have to raise the car up quite a bit to get the tire up into the wheel well. I have about 1" clearance on the inside.

http://www.chevelles.com/showroom/Brob/PortofinoTC.jpg