It will bolt up to a 65-66 but is 2inches wider than stock overall. Max wheel backspacing on a stock 65-66 is 4.75in. If you use that rear you'd have a max backspacing of 5.75in. You do not necessarily have to shorten it.
I put one under my car. I just got new wheels with 1 more inch of backspacing. If you got this rear and wanted the late model 17" bullitt wheels, I believe you wouldn't need wheel spacers (for the rear). Also, that pinion cover plate could be worth something if it's out of a GT car.