65 Mustang...Moving??? Leafsprings inside of frame rails?
Con, how're you going to do that? And, why? Assuming you're trying to install a narrowed rear and bigger tires (inside the stock quarter panels), you'll be doing a heck of a lot of cutting and welding. Wouldn't it be more cost effective to "back end" the car with a pre-made set of frame rails? You're going to need to fabricate some kind of mini-tubs to move the inner wheel housing inboard and you'll still have to deal with the original rear frame.
Of course, if my assumptiuon is incorrect then none of these questions matter.
2 - 65 'stangs, 79 F100 SuperCab, 03 Expedition
Henry Ford said, "If you find you need a tool, you have already paid for it."