Late 80's I was able to swap out pad on my '70 Challenger while frame was still in car. Mine was non AC so there was a little more room to work. AC box and all that goes along with it may really complicate things . Took out front seats and a 1/4" wobble extension will help. Put your chiropractor on speed dial.
Also on my '71 charger the dash can be R&R with windshield in place. Just don't drop any screws down defrost vent. Don't have any experience with '70 and down B's.