leaving the stock rear end and transmission leaves a brownie box as about your only option. I also have a 58 2-ton, original rear and tranny, Mine has a 327 in it. I can roll about 70 down the freeway but the engine is spinning about 4200 RPM. Unfortunately, there is not much magic to going faster, tires need to roll farther per engine RPM, that leaves you 3 options:
1. spin the driveshaft faster (, higher engine RPM, OD tranny, or separate OD box)
2. Change rear end gears
3. Taller tires in rear