Hey Coz,
Fisrt, you know I'm not a mechanic, but my brother-in-law is, 26 years. Second, he got my A-arm bushings out for me today and I asked about your problem here, so here's what he said:
You have 3 possibilities. The ignition switch is bad, the relays in the left panel are dirty or sticking, or your solenoid is bad. It has nothing to do with the MSD ignition because the problem is with starting, not with the engine continuing to run once the ignition is cut off.
It's probably the solenoid. It's usually on the firewall. It has the huge wire coming from the battery on one side and going to the starter on the other side with a small wire attached to it. To get the car started, you can jump the 2 sides of the solenoid with the handles of pair of pliers, with the ignition ON. Not practical in a Pantera. To test the solenoid, get your test light and attach it to the small wire coming from the ignition switch. It should light your test light when the ignition is turned on and it should turn off your test light when the ignition switch is turned back to the RUN position from the START position. It's probably sticking, bad or dirty contacts.
To find your solenoid, trace back from the starter along that fat wire to the solenoid.
There's no reason you gotta go through this, Coz. That bothers me when I'm a long way from home and have car troubles without even simple tools. Tell you what, call me, I'll give you his name & number. You can't call him at work, but you can work out the details with him. He'll give you a freebee, even after this, and give you a discount for 'Professional Consulting' to help you understand your cat way better, and keep you going, from anywhere. May have to get to an auto parts store for a minor tool and part, but it's security. Gotta know the right people. Makes life a lot easier!
Maybe I can help him and you both, but that's between you two, if you want.
Regards,
Ron