More research...
Last night I was looking on the interwebz.
Discovered that 87 - 88 MMs may have come with a 3.55 to 1 rear differential. There were 2 different gear sets: 3.91 (I have) and 3.55.
This would be the perfect ratio for what I am looking for. With a KM 132 5 speed and this ratio, maybe I can drop 1000 RPM at 75 MPH. Yes?

From what I can see, if I drop the center section (carrier) out of a MM with this ratio, it looks like it will bolt into my 79 axle housing.
Anyone know different?