Check the teeth on the ring gear. See if there are any that are worn.

Also you may need to pull the stater and bench test it to make sure it engages and spins freely.

Have you been able to start this truck with this starter ever?

SWEET
Sergeant At Arms: Old Dominion Stovebolt Society
BUNS?!?!?!Where we're going, we don't need no buns.....
1950 GMC 450
1951 Chevy 1/2-Ton
The GreenMachine
In the Stovebolt Gallery