Dart rear tires

how hard is the spring relocation, I have been thinking about this for my 70 Dart Swinger? Chances are I would take it to a local speed shop, but for kicks and grins, I figured I would ask.. I was thinking about the mopar performance kit from jegs (mopar part # P4876558AB) its about $117 for the complete kit..... I dont think I am ready to tub it just yet, but I would like to get at least 10 inches of rubber under each side (or is this possible stock, if so what size tires and what back spacing)?!