A and B bodies used different spring perch angles. If you kept the angle the same when you moved the perches you'll probably need to shim them to get the right angle. And a 2" lift spring will also change the pinion angle quite a bit.
With a 65-67 B-body rear you're going to need about 5.75" of backspace to clear 275's on a 15x8 if you put the car at stock ride height.
The rear came with the car soon as I looked at it I knew something was wrong. Not to mention the springs were shot, no bushings, missing binders. I got a good deal on the springs from a local guy who was cleaning house.
While driving I was thinking about swapping out the 17" Bullet wheels from my 70 Duster with 8.25 to the 74 and put the 255 cop wheels combo from the 74 to the 70...