Probably the best and easiest way to change out the back is to get a hold of Dr. Differential a Moparts member and buy new axles from him with the 4 1/2 inch pattern. Better axles than stock too.

1970 Duster
Edelbrock headed 408
5.984@112.52
422 Indy headed small block
5.982@112.56 mph
9.42@138.27

Livin and lovin life one day at a time