Here is probably your problem........most ignition switches will cut out putting out juice out of the Ignition post when you turn the key to Start. They expect you to pick up the 12 volts to the coil from another source during that brief period when you are holding the key to the Start position.

The way Chevy does this is to utilize a second small post on the starter solenoid. First of all, does your starter solenoid have one big terminal and two small terminals, or just one big terminal and one small terminal?

Answer that question and then I will explain what to do in either case.

Don