The axles aren't usually the weakness in an 8 3/4, the pinion tries to climb the ring gear and breaks teeth off. IIRC Dr Diff has the gears to change to 35 spline axles.
My 408 broke the 8 3/4 in my Dart on the second pass it hooked up, the Richmond gears had about 60 passes and a couple thousand street miles on them and new bearings. This was on a 275 radial. I rebuilt and sold the complete differential, also had to take the transmission out and apart for inspection.
I moved the springs into the frame, mini tubbed, bought new wheels and tires then ordered a new Strange S-60. I went from a 275 to a 325 tire and from 4.30 gears to 4.10's and the car picked up everywhere form the one clean pass it did make with the 8 3/4 with no other changes.
My rear end is moved back 1" and has scraped the stock gas tank a couple times just enough to scratch the cover paint.
