.75v drop through bulkhead??

No, as your first post indicated, if those conditions are as you say, you have too much drop.

Measure like this, and here is what you are measuring: The drop from the battery, fuse link, through the bulkhead, through the ammeter circuit, through the ign switch connector, the switch, and back out it's connector, and back OUT the bulkhead on the dark blue

Turn the key on, engine off. Put one probe on the battery + post, and the other on your dark blue on the ballast. If you have points, make sure the points are closed (drawing current.) The resultant reading will show you directly the drop in this circuit.

Also note that this drop WILL CAUSE overcharging by that amount, IE a .5V drop in this circuit will "tell" the regulator it is 1/2 V low, so it ramps up the alternator another 1/2 volt