Whoa good call Nosser. If you looked further up at the brakedown diagrams, the one for the 8 3/8" (same housing as the 8 1/4" but different guts) that is evident.

an 8 3/8" (truck 8 1/4") can swap the ring and pinion, and 'sure-grip' to the 8 1/4" and I also hear that the bearings can be used as long as the original races go with it....

-D.Idiot

Right DI, but what I meant was you couldn't use an 8 3/8" ring gear with an 8 1/4" pinion.