In all my too many years playing with cars I have come to realise that when buying replacement parts always go for the heaviest duty one available even though it may cost more because in the long run it will have more reserve rather than working at its maximum limit.
With starter motors always get a heavy duty double reduction drive one rather than a direct drive model.
It doesn't have to work as hard and also doesn't draw as much current from the battery therefore saving wear and tear on both starter and battery and in turn frustration on your part.
Bill, sorry to hear of your starter troubles. A couple of ideas:
1. take the failed starter out and try to inspect the ring gear very closely. Get a proper inspection light and one of those small extendable mirrors and check it out closely looking for broken teeth (turn the engine through 360 degrees) and wear spots.
2. Depending on the type of starter you have, take the starter apart and merely bolt up the face to the block - see if you can extend the starter gear and "feel" if engages with the ring gear without the starter gear being cocked to one side or binding. You may just have some misalignment but it sounds to me like you have either a) broken/missing ring gear teeth in a position of common repose for the engine (low compression), or b) misalignment to the extent that the starter gear is not actually engaging the ring gear.
It'll become clear with some more inspection and monkeying around with it - don't give up!
I think that either there is an electrical or mechanical problem where the starter solenoid is not releasing.. With all the racket of the solid lifters and loud exhaust, you'd likely not hear the starter still engaged.
I think when this happens it strips one of the gear reduction gears inside the PMGR starter.
I'll call you later and we'll go over some electrical tests..
It might be something as stupid as a solenoid getting hung or a lazy switch that does not release all the way from starting contact.
Bill; I think Randy is saying that if you don't do these electrical tests the new starter might fail as well.
***EDIT*** Sorry Bill; I saw on your other post that you were going to replace the starter button and therefore this post is not useful. Good luck with it; hope you get it up and running soon. You have my favorite GT40 on this site.
Since you have had 2 starters that fail the same way the problem may be elsewere. If your drivetrain is like mine there is an engine from one company, a trans from a second company, adapter/flywheel from a third company and a starter from a forth. So things might not always line up properly from no fault of the individual parts. Find out what the common problem is for the starter failures. Are the 2 starters the same brand or if remans did they come from the same rebuilder. When you say the starter is spinning is it the armature and not the gear or is the gear not engaging and just spinning. Maybe more investigation is in order. Can the local Pep Boys check the starter?? It might be something other than the starters.