Biggest rear tire in a 68 Dart?

What is the biggest tire I can fit in a stock wheel well of a 1968 Dart assuming I have the perfect backspacing for the tire? I am thinking either 15" or 17" wheels. I don't want to mini-tub it, but may do so later if I have to.

What about a spring relocation kit for the back that Mopar sells? What has to be done, is it a bolt in, or do I have to do some cutting?

I already have a 8 3/4 from a b-body and 5 x 4 1/2 bolt pattern. I will have a custom set of wheels built to fit.