If you are sure that tang is not turning then you got trouble. Usually that shaft has problems with not staying "keyed" in to the pump and distributor. Make sure that is not the problem. That is driven off the center of the camshaft.
Look under the truck and see about getting the oil pan off. Maybe you can unbolt the motor mounts and lift the motor a bit to get the pan off. Then the oil pump comes right off and you can see the gear on the center of the cam too.