First of all... These old girls will go straight down the road at 80 mph all day long. Please make sure the brakes are up to the challenge and the suspension is in proper spec.
3.38 gears will not drop rpms as much as an od transmission. The change from the stock 3.90 gears to 3.38 gears will give you about a 15% drop in rpms. The od swap will give you about 27% drop.
3.38 gears are not the only option. 1957-1959 gmc 1/2 ton trucks with the 3 speed had a 3.08 spicer rear. This rear will bolt right in to a chevy, but parts like brake drums, gears, and cables are hard to find for the spicer rear.
The od transmission swap is likely the cheapest and best swap. Using higher gears will affect the performance in every gear. With an od, you get to use your low gears for acceleration, and then have an extra gear that will shed at least 25% of the rpms at cruising speeds.
I vote for the od transmission. I have had a 59 gmc with 270 6 and 3.08 gears. I have had a 59 gmc with the 3.90 gears and changed them to the 3.38 gears. I now have a 1959 gmc with an od transmission and gears a little lower than stock for better acceleration. I have been there and done that... In my honest opinion, 3.90 gears with 28-30 inch tires and an OD transmission will give very rewarding results. Find an online calculator and run some numbers for yourself.
