3.5 swap?
Good luck cutting the IRS down. It's hard to tell from the pics but it looks like sectioning the subframe may be your best bet. Thus keeping all the stock arms and geometry. Obviously, you'll need to shorten the half shafts.
Take lots of pics, I am putting a 6.1L Hemi into my '67 Dart, I had been planning to use either an 8 3/4 or a Ford 8.8. Either way, I'm going to have at least 1000.00 in the rear. If I could simply section a rear out of a Charger I'd gain disc brakes and something unusual.
Hmmmm,
got me thinkin', always dangerous.
Regards,
Joe Dokes