You say that you have a "ground to a bolt that mounts the unit to the tank." What is the other end of that ground wire bolted to? It needs to be attached to bare metal which is a part of the grounded chassis (frame). I will bet 12 doughnuts of various flavors that is your trouble.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission