I probably thought about it way too much. I knew I wanted a new Magnum for strength and I was already planning to cut the tunnel for easy of service. I loved driving the car with the Muncie for so many years (with 4.56 gears) that I figured it would be fun to pick a transmission and rear ratio somewhat close to the original gear ratios. I know it is weird but I used to enjoy starting in second and then just shifting straight to fourth with the deep rear ratio. I built a little gear calculator and messed around with the numbers.
The wide ratio (TUET11012) first four gears lined up closest to the original Muncie wide ratio gear ratios, coupled with the new 3.90 rear gear.
Then I looked at highway speeds and figured I want to cruise the heck out of this car and the roads are long and straight out here in the west. It was easy to select the wide ratio with the .5 final ratio to keep the RPM below 2,000 at 75 MPH.
If the car runs out a steam too early on the Auto-X then I can always select a higher gear ratio like 3.70 or 3.5.