I don't think it's a "bolt in" deal but it's a lot easier than moving the springs into the frame. :D If you're looking at the Prostar then I'm guessing you have switched to the large bolt pattern. Summit has a wheel that's similar with 4.5" backspacing on an 8" wheel. That might work for you, but if you want to put a 275 tire in the back it's going to be really tight & I think you're going to have to measure VERY carefully. Go to this site with the measurements from your current wheel/tire combo & play with it a bit. It's been a real help for me.
http://www.rims-n-tires.com/rt_specs.jsp
I've been wanting to put a 275 tire on my car & it looks like an 8.5" wheel with about 4 7/8" to 5" backspacing (3mm to 6mm positive offset) would be the way to do it. Your car might have a bit more room on the outside than mine. I think I'm going to look at using a 265/60 or 265/50 15 in the back. A 10.5" cross section in a wheel tub that's only about 11.5" to 11.75" inches wide [-o< is about as far as I'm willing to go.
:burnout: