Yep that's it. Trucks where limited to 45 mph prior to 1960.
You 2-speed axle will only give you a lower low. You could get taller tires and the 2-speed axle then you'd still have the low, but you'd get a little more speed out of her at the top end.
The brownie (spicer auxiliary transmission) I've got has a 0.8 overdrive gear. That gives me slightly longer legs. I've still got the SM420 with only 1:1 4'th gear. Most spicer trans that I've seen have only a 1:1 top gear, so even if you get a 5 speed, it'll only give you more low, not more high.
Now if you're never going to haul anything (and I mean NEVER) find yourself a Dana 70 or a GM 14 bolt axle and swap in that rear end. You can get low ratio (3 something) in these axles. That'll get you speed... but they'll never stand up to the carrying capacity of the rest of the truck. It'll look wierd under that classic truck as well.
Is this a tandem axle? If so, why not look for a Mack or Kenworth or whatever tandem in a truck salvage lot. I think you can get 4.11 gears on these. What is the bolt pattern of your front axle? Can you find an old Mack rear that matches this bolt pattern? It's just a convenience of having the same bolt pattern.
