It's not just the contacts it's the entire voltage drop from the battery all the way "up" through the ignition switch, back to the rear through the battery switch, back up to the front to the ignition/ alternator.
You could split off the load, all it has to do is either kill the alternator field or ignition but not both. If you use no14 or overkill 12 wire you should be fine.
This would be a GREAT time to see if your battery/ regulator is seeing the actual battery voltage "sense" for charging. Access the wiring as close as you can to the VR IGN terminal. This might be coil + in your case with HEI. Turn the key to "run" engine stopped. Measure battery voltage right at the battery posts, then run up front and measure VR IGN terminal voltage from there to VR case (mounting bolt)
The two measurements should be within .3V (3/10's of one volt) "Likely suspects" for drop are the bulkhead connector, ignition switch, and switch connector
You can double check this running, You want the engine warm and running, the battery "normalized" for charge. Measure battery charging/ running voltage at fast idle, it should be 13.8--14.2, not below 13.5, and not above 14.5
