Grind off the old perches and do a mock-up on your 71...IIRC the mopar performance perches are the ones you DONT want... They had that bigger center locating hole like the set you're taking off...As stated it's an easy fix just make up a spacer but why start with them if you can just get a better set...

Get the new perches,set them on the springs and install the dana on them...Bolt the U bolts but leave them loose until you measure your wheel openings to center the rear...Don't worry about the pinion angle at this point just check how the rear fits with the available spacings...A custom wheel offset is easy enough to do and you'll fill up the wheelwell much better in the process...

The troublespot is always the inside tire sidewall at the rear leaf section the front segment before the rear housing gets narrower so if the back segment clears the front will by design..