Starter in a 1974 Chevy Truck

Make sure you have the right starter. Make sure it is properly mated to the block/transmission. Make sure there is nothing contaminating the mating surfaces. If possible, remove the inspection cover and watch what's happening as you attempt to start the truck. Remove the coil wire to prevent it from starting, you want to see how the starter is engaging the teeth on the flywheel/flexplate. Make sure the teeth on both parts match up properly.