There was a .76 fifth but it isn't as common, some people don't know about it and some may even say you're wrong. But you aren't. It was used in S10 trucks with a V6 in 84 & 85 and the gear chain began with a 4.03 first. As I recall, 3 id numbers had that 5th gear ratio. Let me know if you need them...I have a list somewhere here.
Back to the idea of driving a 1938 pickup at freeway speed, others have mentioned this but a friend of mine is dead and has been for over 60 years because he thought he was smarter than GM engineers. Please understand you own a truck for which the suspension and steering was designed 85+ years ago. This is not a bad design but never in any way did GM consider anyone would be driving at 70 or 70+ mph for extended periods of time. They realized somebody might get the truck up to 70 but they did not expect it to be anything but an infrequent occurrence. I should add my first car was a 1936 Ford. It only had 30,000 miles on it when I got it. At any speed over 60 it would wander, veer and shimmy so much you needed to carry extra underwear with you. Like your 1938 truck, you could only make a couple of adjustments to that front end and new parts only did so much. The deficiencies of handling were built in to it. It also had mechanical brakes which were just a bit better than dragging your feet to stop.
The 1938 Chevy truck he put a V8 and different transmission and rear end into would easily drive at freeway speed but it couldn't steer safely, couldn't stop safely and here is the really important point: not one thing inside that cab or about that cab was designed with the idea of protecting the occupant in a high speed crash. Not the steering column, the steering wheel, the glass panels in the 3 gauges, the bracing, the swing-out windshield apparatus, the way the doors attach and lock, the way the seat mounts...nothing contemplated a high speed crash. In his crash everything flew apart instantly and either became a weapon or left parts of him exposed for severe injury.
They thought my friend was probably going 70 to 73 when he lost control. Measurements suggested that. It was on a 2 lane flat and very straight section. At that time of day the sun was at his back. The truck rolled over so violently it looked like somebody had pushed it out of an airplane and onto the side of a rocky mountain. They said parts of him and the truck were scattered 30 yards. The ambulance driver told me he figured it took less than 5 seconds from the time he lost control until it stopped rolling. At 70 you're covering over 100 feet per second, by the way.
Years ago...it is gone now...there was an internal memo on the GM Heritage Pages that advised dealers to instruct people the AD pickups (which were designed years after 1938) had been engineered for a safe and prudent highway speed of 55mph. My honest suggestion: if you want a truck you can drive at freeway speed, go buy something built after about 1963. Stay safe...you have one life and it is better if you hang around a long time.
