I totally missed asking you which engine you have (will have) in your 72. Makes a huge difference.
It's like a seesaw, you have to find the balance that you need. The rear axle ratio is important to to determine 1st gear launching and final drive OD. At the same time you have to factor in the "sweet spot" for the engine's torque/RPM curve for highway speeds. 4th gear output of the Sag is 1:1. The R10 B-W unit has a planetary gear set that fits in between the transmission and the tail shaft housing and offers a .70:1 ratio. The R10Q-1 has a short tail housing and when bolted to the Sag 4 speed, you can reuse your drive shaft.
Very rough idea of what I'm trying to say:
3.11 X 3.73 = 11.6 (good first gear launching) 0.70 X 3.73 = 2.62 (tall final drive, probably too much for the engine)
As crenwelge mentioned, the B-W OD was used with 3.90 and 4.11 rear axles. This design was to get better gas mileage beginning back it the 1930's.
0.70 X 3.9 = 2.73 0.70 x 4.11 = 2.87
Now after all this. You can also work up a combination or rear axle ratio and tire diameter to get the equivalent of an OD unit with your stock Sag.
The attached picture, not mine, is a rough example of a shifter application. The other picture is a Patrick's shifter (there may be a design/shifting issue with using the very short shift rods).
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
