I am pretty late to this thread but,

I bought off the showroom floor and from the man himself, Burt C. Hawk of BC Hawk Chrysler Plymouth in Bellevue, WA a 1971 Plymouth Valiant Scamp that was factory equipped with the 225 Slant 6, 727 Auto and 8.75 rear. It was a daily driver with factory rally wheels, E70x14 Goodyear Polyglass and factory disc brakes. I had to give it to the ex-wife as part of the divorce in 1977 and her boyfriend ran the engine out of oil and it kicked a rod out. It ended up in a wrecking yard in SoCal.

I miss that car

I never think about her

Here's some comparison pictures.

My 69 belvedere slant 6/3 speed manual has a 7-1/4 rear. First one I had over the years that came with that rear.

Local drive shaft shop recommends 1 inch of yoke showing with a dust boot, If it has a dust boot and the yoke basically touches it the shaft is to long. my bet is you will need to shorten the shaft going from a 8 1/4 to a 8 3/4