I've bought a new starter when I swapped out the last ring gear. The starter is definitely what's cutting off teeth. The starter/ring gear geometry shouldn't be any different than it was with my last engine (440 blocks are all basically the same). I didn't have to shim the last starter, and I didn't shim this one.
Again - I keep thinking that the problem lies in ignition...

"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson