You may have had an electrolock coil distributor set up. If so, you need to revert to a normal distributor and coil. Find a NEW aftermarket switch that fits the hole and looks like you want it to look from the front and has a key tumbler with key. Does NOT MATTER how many terminals it has on the back. A simple ON/Off two position switch is usually hard to find. May have to settle for a three postion and just use two. No big deal.

Let us know about any funky, non original things you are adding or if you are going 6V or 12V. Your GMC would have been positve ground. You should revert to negative ground.

Are you foot start and have a 228??????? Fill us in when the time is right.

Pictures of all your ignition parts you have and the dash, would be very very helpful as well as the few questions I"ve asked.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn