old 46,
The 81 3/4 ton rear end will be wider than the 46 rear. The newer rear is 62 to 64 inches wide. New spring perches will be needed as well.
Along with the rear end, the you would need to change the tranny to make it all go together.
If you are running a truck with box and fenders, the 81 rearend may not clear your fenders too well.
