Classic/Antique Car Repair: Overdrive conversion (Packard , 1939 120 std 8), overdrive units, overdrive transmission

Question
1. How much of a job is it to add and overdrive to this drivetrain?
2. What Packard overdrive units will fit, the later the better?
3. Any changes to the drive shaft?

Thanks

Answer
This is a major job.  The transmission and driveline have to be changed, along with adding the electrical and linkage controls. You need to also change the rear axle if you want to get all the benefit of the overdrive, the ratio in your car is taller than that used in the overdrive cars.   

Since these cars were available with overdrive all the way up to 1947 with the same engine (the 282), you should be able to find a parts car with all the required parts, including the added rear support for the overdrive case and the correct rear axle.  

The overdrive used in those years was the R9 - I don't advise trying to adapt a later unit.  There is nothing wrong with an R9 if it is driven sensibly. (I know, I've been driving my 47 Custom Super Clipper with an R9 for almost 40 years), especially since you have the smaller engine.  If you do this,  NEVER run the car in overdrive in any gear other than third - the R9 cannot take the torque of a straight 8 in second overdrive.  The driveshaft has to be shortened, because the overdrive transmission is about a foot longer than the non-overdrive unit.

Dick