The aftermarket ignition switch I bought for my 73(Advance auto) is designed to fit multiple applications and has an extra ground wire in the plug. On my 73, the ignition switch side of that plug was unused. The main dash harness side of that plug however, has two orange wires in it that have power. I had to remove the extra ground wire from the plug and tie it off out of the way. Check to make sure that's not your problem.

You can also take apart the ignition switch connector and test for continuity between the red power "in" wire and the blue "ignition run" wire. You should only have continuity when the key is in the "run" position. That would isolate the switch if that was the issue.