9" vs 10" drum brake spindle question

Actually, Doctor Diff does have the 4" BC option on that kit... It consists of late rotors drilled for the 4" bolt circle, and (I believe) the hub turned down to the smaller register. But it's nothing you couldn't have modified locally if you're in a bind for replacement parts.
Thanks, I thought maybe, but it was late, my eyes were blurry and I was using my phone. But you're right, easily available stock parts could be easily modified after Cass retires.

I think the 9 and 10 use a different lower ball joint don't they?
Yes. The 9" brake ball joint is light duty and isn't really suited for "spirited" driving. It's fine for it's original purpose though. There are at least four different lower ball joints for A-Bodies, so do a little homework to get the right ones.


One other issue with the later spindles used in the Doctor Diff kit is they move the hub out a fraction of an inch. For most, this isn't a problem.