I probably bought wrong 8 3/4

Help! I have been doing a full restoration of an original CA owner '65 Dart GT for six months now. I have aquired every option available (pwr. steer, front rotors,full 235 h.p. commando rebuild etc.) and have loved every minute of it, until now. About three months back I aquired a complete 8 3/4 rear-end from a guy who told me all I had to do was shorten the driveshaft. (sucker that I was) Well now I get around to reworking the rear end and ordered the full spring 'n things hookups and I'm senseing that the flange to flange (around 57 1/2) is not going to fit. (I know..Duh) I see faint salvage yard paint markings "Dart '74" and if this wont work without cutting the case and axles it's going back on Craig'slist. Should I dump this one and look for a specific '66 thru '72 Chrysler of some kind to scalp or give up and stick with my silly 7 1/4 with no suretrak. It's important to note that I don't intend on gettin on it as much as I like what I've heard about beefing up the diff. Thanks, Frank