Ideally, you would want to see it with battery connected. That could be dangerous without proper precautions. On a lift is safest way to observe what happens when it spins with no engagement. As Jerry says, if it spins it is not "slipping" on teeth. It seems like the starter drive is not fully engaging. A visual will confirm that when it spins, it's just not engaging the flywheel.
If unable to observe with live power, you will have to take off starter and take to the bench and test....or replace drive (Rockauto 9 tooth: $10 and up) and reinstall to see any change. Inspect flywheel teeth all the way around so as to eliminate it. If it's beyond your expertise to replace starter drive, buy a whole rebuilt starter ($160 Classic Chevy Trucks) if flywheel and linkage are OK. Buy the parts wherever you want.
Be sure you understand how the foot stomp works. Be sure that a sloppy pivot or link is not working sometimes and then aligns and works other times.
1. What does it do when it does not engage? Spins, growls, grinds, clicks, spins fast? Tries to start truck then gear pops out?
2. What does it do when it engages? starts truck right up and disengages without grinding? after-start grinding/spinning?
3. Any slap happy noises from foot start mechanism? You DID NOT say what you found by examining the foot stomp mechanism from above (battery disconnected)
4. Does the foot start mechanism still have it's return spring attached. (not talking about starter drive spring).
5. Can you see a starter drive spring on starter shaft next to pinion gear?
6. Can you see if pinion gear teeth are OK except for a lead in chamfer on it's edge?
7. This takes grunt, sweat and tears to examine all this. Flashlights, mirrors and fingers.
8. When checking with live battery, you must pull center dizzy tower wire so that motor won't start.
