8 3/4 B to A swap

To summarize what's already been said, the best B body rear to use in an A body is the 65-67 B body rear, followed by the 68-70 B body rear which is roughly 1/4 inch wider.

Even the 68-70 is only 3 inches wider which translates to 1 1/2 inches in extra width on each side.

More importantly, the center to center width on the spring perches is 44 inches as opposed to the 43 inches on stock A bodies. That's 1/2 inch on each side.

Some people move the spring pads in 1/2 inch. I do not. If you pull sideways slightly on the leaf springs, the 44 inch spring seats wil drop right down on the tabs. The 66-70 B body housing is virtually a bolt in swap.

You could be way ahead of the game both financially and stress wise to sell your 73 housing and buy a 66-70 B body instead.

This may help you as well:
http://www.moparmusclemagazine.com/techarticles/drivetrain/mopp_0404_8_3_4_rear_axle_housing_swap/viewall.html

(although from experience I would dispute the need to move the spring seats or enlarge the holes)


Good Luck!