I sprung for the 3.38 gears in my 59 gmc. IMHO the 3.38 gears are a poor compromise. Granted some gmc 1/2 ton trucks of the era had the economy 3.07 rear, which is a higer gear ratio, and the 6 cylinder 270 had no problem moving it around in town.
I personally feel a 5 speed transmission is a better compromise... you get all the benefits of the low gears, and even more economy in OD.
3.38 gears will work with an OD, but it is not a desireable setup. The stock 3.90 will work much better with an OD transmission and a 235.
If you're happy with the 4 speed and want more economy, go for it... just be aware that you won't have as much low end grunt, or as much economy as you could have with a OD transmission.
