What is the difference Mopar 8 3/4 to 9" ford?

The 9" has a smaller pinion diameter than all of the 8 3/4's (741,742,489). The 9" has three bearings supporting the pinion were the 8 3/4 has two. The 9" takes about 5% more HP to turn that a 8 3/4 because of the pinion location on the ring gear. The ratios for ring and pinions for a 9" are just about endless while there are not near as many for the 8 3/4.

There is nothing interchangable from one rearend to the other.
Chuck