After messing with the ignition problem all day, I MAY have gained some progress, but I won't know until I get some miles on it.
What I found was, no matter what I tried, I kept getting a very weak erratic spark when jumping the trigger wires. I tried moving the ground to the battery negative cable directly and no change. I thought maybe the extension for the trigger wires may be faulty or have a bad connection since I left the old one in place, so I removed it. I noticed a bit of corrosion on the small prongs in the connector, so I was feeling somewhat confident that that may be causing an issue. So I replaced it with the one that came with the new box....and no change. Still very erratic, if I got a spark at all.
Feeling a bit flustered as to why this was happening, I considered the fact that the trigger wires ran parallel with the coil wires. I wondered if there was some interference going on there, so I ran both the old and new extensions from the box, outside of the car, across the engine compartment and finally to the distributor. Suddenly I had a good strong CONSISTENT spark. So I removed one of the extensions, rerouted the wires through a different hole in the firewall, down the right side of the engine (opposite side of the coil wires) and hooked it up. I still had a good strong consistent spark.
The car fired right up, but I have no way of knowing if that solved the problem yet. I was going to go for another test run, but now rain has popped up, so it'll have to wait until tomorrow night.
I made a short video showing how the spark was acting. While I typically got some erratic spark while jumping the trigger wires, I was getting nothing while I shot this video. But you can see that I was getting great spark whenever I grounded the trigger wire to any part of the chassis or engine. There's nothing in MSD's instructions that says the box must be grounded, and based on my findings after rerouting the wires, it doesn't seem to need it. I have my box mounted under the heater box inside the car, and it has ran that way flawlessly for over 15 years, so I'm still puzzled as to why I'm suddenly having this issue.
I have one working theory; that the old box was not affected by the wires running parallel. But the box went bad after 15 years. Then, with the new DIGITAL box, it MAY have issues with the wires running parallel. It's a long shot, and I'm not even sure that I believe it, but I have no other explanation at this point. Anyone else willing to take a stab at a theory?
The video showing the spark problem...
https://youtu.be/85PLiA8qIr0How my box is mounted...
Finally, a shot of the installed differential with 4.30 gears.