I had to do the following on a 1969 chevy 1 ton with a 350 w/ stock headers. There were two small wires (wire A and wire B) AND the "big" hot wire to the battery on my starter. If you jumpered the "big" wire to wire A, the starter would spin. Wire B is the wire I had to clean all the terminals & replace. I also had to put a BIG ground strap from the starter to the frame.
P.S. I think 64fleetside is the closest to explaining what's going on. I think most things electrical are smoke and mirrors
