How difficult is it to swap out a dashpad in a 71 challenger?

Kind of a pain, but doable with patience. You need to lower the steering column,remove the cluster and switch panel. Also remove the glove box and ash tray. 1/4 inch ratchet and sockets make it easier. Having the new pad laying there to see where all the studs are located helps if you've never done it before. Don't forget the screws on the sides! Good luck.

sometimes you have to remove both of the pillar post panels since they touch the sides of the dash on top

If your going to do it in the car...it's a PITA, so much easier to pull the dash assembly out of the car, yes it may seem daunting if you've never done it before, but IMHO you risk damaging more componets/trim fighting an incar exchange of the pad, I've owned and restored E bodies for over 30 years, I've only changed a pad once "in" the car, never again!......4 screws in the defroster vents, 1 bolt in each lower kick panel corner, a few bolts to drop the column, harness disconnect, a few cables(speedo/vent/heat) to undo and the entire assembly can be sitting on your work bench, kitchen table, etc....so much easier

I would suggest pulling the dash and reinstall with new pad attached to frame. If you do this, it is a two man job on the reinstall. Those four screws at the defroster location are a bear! Be carefull freewheeling with your ratchet near the windshield. Ask me how I know.

I agree, pull the whole dash. Only about 6 bolts and unplugging harnesses. I just got done with mine and couldn't believe how easy it all came out. good luck I just followed anlauto posts on this thread. https://board.moparts.org/ubbthreads/showflat.php?Number=5524548