I'm considering an A 500 4 speed overdrive for a 72 Duster

There are several threads posted on here about installing them, and the crossmember rework you need to do.

A few old threads

42RH / A-500 in a Duster with pictures.

A500 swap help.

switching from 904 to a A500 or A518, anyone done this?

9 Months after the A500 swap from a 904