Just a random thought: If you can turn the engine by hand easily and the flywheel turns when doing so then the problem has to be in the starter. I know you said you tried a different starter with the same results. Perhaps it needs shims, It may be meshing so tight up against the flywheel that it binds the starter and won't allow it to turn.
EDIT: I notice that the starter out of your 52 should be a negative ground starter. Does hooking it up backwards make it turn a different direction? I don't know the answer to that question.
Last edited by FriedGreenTmater; 10/22/2010 4:40 PM.
