Spence, Faster rear gears, with the same 10 lug on 7.25" bolt pattern, can be had from a 3500HD, or a P30 chassis with the 10 lug rear axle, best bet is a Dana 70HD or 80, as you have faster gear choices up to about 3.54. Some of these possible donor trucks used a Rockwell rear axle, spot it because of the square axle tubes, these have less of a gear selection, and why the Dana is the better choice. All will be disc brakes, so changing the front for disc brakes at the same time is a good idea, almost mandatory.
