The Ign Sw provides a ground to the distributor. Depending on the type of switch you use, just ground the case or the second terminal. If you'd like to convert the car to switched ignition through the ignition sw, run the switch to a subminiature DPDT relay, then off one set of relay contacts you have the switched ground for the ignition and off the second set of contacts you can get the switched ignition to run accessories. A 5volt submin DPDT relay off of ebay or amazon is only a few bucks. Easy enough to hide it in heat shrink tubing by the wiring harness, however make sure you make a note of the changes and tape it by the relay or document it somehow so the next person isn't expecting stock wiring.