Voltage drop in crank

The FIRST thing to do is actually check the voltage. There are only two wires, easily traced, and you need to examine the load you have on it all You don't need a second person you need these wonderful inventions called alligator clips LOL

1....Even with stock wiring, I recommend you add several relays. This depends on the loads on the ignition feed, and maybe even accessory feed. So 1 for high beams, 1 for low beams, and at least one triggered by "ignition run" to feed underhood loads.

2...There is no need for any diode. Jumper the IGN1 and IGN2 together and use that pair to trigger the underhood relay. Feed the power in to the relay from a nice large say, no10 and a big fuse or breaker. If you still have the factory starter relay "the big stud" is a good place.