Narrowing an 8 1/4" rear end, how is it done?

I had a 74 Duster crate 380hp 360 4 speed and a 8 1/4'' 3.21 rear.It held up ok meaning I never broke it.
Axle tube ends are the same as the 8 3/4 so no problem buying those.Same with new perches.
Axles I have no idea if the splines are long enough to cut down. It may be possible to get custom length.
Hope your local Driveshaft shop does differentials too.