All I did was put a terminal strip on the firewall with tongue type connectors. You know the ones with holes in them. The strip had number 8 machine screw studs. (We used them a lot in my old avionics job) And I ran all the wires to that point and made a connection there. (Except the unused Chrysler wires were just stowed there for emergency)
This way I could swap wire connections if needed.
If the MSD ever gave me a problem I could just remove it and power up the necessary wires for the Chrysler system.
I carried prints that I drew up (I modified the Chrysler ones) with step-by-step instructions for moving wires based on terminal numbers.
Then I wouldnt have to try and remember how to move the wires around on the side of the road if I needed to. Fortunately, I never had to use the back up with my replacement MSD box..
But then again I did'ntt drive that car very much.
When the car got hot the original MSD wouldnt run. But I could swap it over to the Chrysler system and it would start and run.
The Chrysler would work and not the MSD. I have an idea why, but it is just a theory. I do know this modification worked for my piece of mind.
I sold that 340 Challenger several years ago. I could look around and see if I have some copies of the prints I drew up and email them to you,.
They might not help you specifically for your Excel but they would let you visualize what I am trying to say and see how to do it. You would have to noodle the particulars out for the Excel. I am not familiar with the Excel unit.
But I would imagine that you would have to completely disconnect it from your system and connect the Chrysler system up and vise versa.
Your car certainly wouldnt run them both at once.
It is just a few wires and easier than it sounds.
Again it wasnt just about the cost to me, it was that wrecker and hassle thing.
(Although I admit to not wanting to buy another MSD for that car which I might NEVER need)