I seem to remember some older powermaster starters were shipped with the wrong gear for AMCs. It might be worth a call to powermaster. Did you keep the plate between the block and bellhousing? It has to be there to get the starter depth right. When you replace the ring gear, put the flywheel in the freezer overnight, then put the ring gear in the oven for a while, the gear will drop on and tighten as temps equalize.

Bob
tufcj