On most ignition switches, the run position sends power the the distributor, points or HEI. On a points system the wire out has a resistor built in to supply slightly less then 12 volts so the points last longer. For starting, GM used a terminal off the starter solenoid to supply the full 12 volts to the coil, then switched back to the run circuit once the engine started.
To get around this with HEI, you can simply replace the ignition wire off the key switch with a 10 gauge wire, and the one off the starter with same size wire. Relays will work, but be sure its a 40 to 50amp relay if you plan to run the engine hard. HEI works best with full voltage. Aftermarket key switches supply ign at both run and start.
For simple testing and breaking in of the engine, go straight to the battery with a simple on/off switch in line. Ignition power doesn't need to be fused, GM never did.
Voltage will drop when the starter is working hard, and if the ground system in the dash is weak, the whole fuse box will loose voltage. A good location for ignition power is the feed line to the amp gauge if the car has one, this wire comes straight from the alternator and is suppling full current.
Joe
