I suspect that an overdrive trans could work pretty well with the little six. The problem I have with Darwin's modified 262 V6 is that the engine is not happy running below 1200rpm or so, the vacuum and fuel efficiency drop off With the 3 speed manual and 2.73 rear gears, my wagon is reasonably efficient above 40mph in third but when I start approaching 30mph (like driving in downtown Wichita) I find myself getting ready to shift into 2nd. My eventual plan is to replace the Saginaw 3 speed with a 5 speed OD and a set of 3.23-3.42 gears so that 4th gear will keep the engine happy for 25-45mph "around town" cruising and 5th will keep the revs even lower on the highway than they currently are.
You might want to play with the following equation:
rpm = (MPH X Gear Ratio X 336)/Tire Diameter
where
rpm is engine speed in revolutions per minute
MPH is speed in miles per hour
Gear Ratio is the final drive ratio (rear end ratio X trans gear ratio)
Tire Diameter is the tire's diameter in inches.
Play with different vehicle speeds and see what rear gear ratio will keep you in the 1000-2000rpm range for most of your normal driving range.
Commonly available ring and pinion sets for a 7.5 GM rear are 2.73, 3.08, 3.23, 3.42 3.73 & 4.10
Have Fun!!!