Instead of swapping final drives, just upgrade to a 1987 300D!
Bunch more power, too (148 vs. ~125, I think.)
The 87's (W124) have something like a 2.27 rearend, which works out to 25mph per 1000rpm. Therefore:
1000rpm = 25mph
2000rpm = 50mph
3000rpm = 75mph
4000rpm = 100mph
It drives quite nicely at 100mph, and yes, the cruise control does work at that speed... (Montana, a few years ago, in case you were wondering.)