I would recommend to you that you contact Jon Haas at Pantera-Electronics.com. He has quite a bit of experience with the Pantera tach.
Stop guessing and trying to read tea leaves. You aren't going to find a solution on the internet. It doesn't exist there.
The wiring on a Pantera is a little out of the norm of what you expect and can give you fits when you delve into different then stock ignition systems.
I am running the Motorcraft Duraspark II distributor along with the Pantera-Elecronics Ignition Adapter. EVERYTHING just plugs in and works as it should.
I tried running an MSD and as par for the course with MSD, was a complete disaster and more importantly a waste of time and money.
You can do it the cheap way and try to screw around with it yourself and still be confused or even MORE confused or you can go directly to an expert that can fix it. Go to Jon.
The Pantera-Electronics system is the only system that works. PERIOD.
"Living in America is really neat. You don't have to run through the jungle and scrape up your feet." - Randy Newman. Of course if you want to scrape up your feet...go right ahead!