hotshoe36,
You have certainly done the right things so far. The obvious choices would be it isn't turning fast enough OR it can't turn fast enough.
It seems to me you have ruled out the electrical end of things by installing a new starter, then jumping the rig with a 12 volt battery. If all was well, that should have make it spin like crazy.
The next thought would be that some mechanical issue is binding the engine up. If this were the case, you would probably see the effects of this problem as you ran the engine for awhile.
I would pull the gravel pan when the engine was cold and see exactly how much oink it takes to turn it over. You indicate it's an old 216 which, with the plugs pulled it should turn easily with the fan.
Back on the electrical end of things, you could have received a bum starter, or your cables could be too small or failing at the terminal (internally).
Did the truck run fine..then this problem popped up! Scout around a bit more. See if the engine is tight for some reason and resisting the new starter.
My bets are with the starting system, not a mechanical problem. Your battery, cables, connections and starter all must be sound for this thing to spin over nicely. Keep us posted
Stuart
