My 315 poly keeps running after everything turned off..

OP you keep saying stuff like "It's properly wired" and "I've checked the wiring," but I and others cannot SEE how this is. We do not have documentation on either the original wiring, or how this is actually wired.

I mentioned the starter solenoid, and still think this is a possibility. DOES THE SOLENOID incorporate a ballast bypass circuit? Can you provide a photo of the solenoid, and identify the wires?

How about the ignition, "what" is it and how is it wired? Are you running a ballast (ignition) resistor?

Have you found an online manual or wiring diagram you can link? Is it wired "as per" factory or is this a custom harness?

I am not bragging when I say I have a VERY good understanding of electrics. I'm 74. But I cannot work "by imagination."