[Sheepdip]

This explains it better than I could

Modified wiring with the remote Ford solenoid
If you are running an GM HEI distributor, don't worry about wiring to "I" terminals.

The wire that goes to the "S" terminal on the GM solenoid (closest to the block on GM engines having the starter on the passenger side of the engine) is removed and wired to the "S" terminal on the Ford solenoid.

The wire that goes to the "I" terminal on the GM solenoid (farthest from the block) is removed and wired to the "I" terminal on the Ford solenoid. The "I" terminal on the GM solenoid will not be used. When you purchase the Ford solenoid, make sure that it has the "I" terminal needed for non-HEI/points-type distributors.

The positive battery cable is removed from the GM solenoid and is put on one of the large terminals on the Ford solenoid.

Another cable is run from the other large terminal on the Ford solenoid to the large terminal on the GM solenoid where the hot battery cable was just removed.

Also on the large terminal on the GM solenoid, you have to add a heavy gauge (10 gauge) wire to the "S" terminal on the GM solenoid, or use a shunt. This can be bought or easily made.


Solenoid shunt for GM starter solenoid
This is what happens when you hit the start switch:

The electricity from the starter switch only has to energize the Ford solenoid (that is mounted in a relatively cool location).

The "battery" cable to the GM solenoid goes hot. The Ford "I" terminal also goes hot, providing non-ballast/full battery voltage to the coil.

The GM solenoid now gets a solid "high capacity" current supply and works the plunger and contacts. The internal contacts send voltage to the starter motor.

This set-up DOES NOT make the GM solenoid any cooler. It just gives it a better current supply. Anything that can be done to shield the starter from the header heat would be beneficial.