If you get an SM420 like Racecarl suggested, you may want to consider a Gear Vendors overdrive. I put one on my '51 3/4 ton and I love it. My top speed is now comfortably 70 MPH instead of 50 as it was with the 4.76 rear axle ratio. The Gear Vendors unit gives you overdrive in each of the top 3 gears (so you have 7 forward speeds if you really like shifting) and the overdrive is .78. I rarely use it in any gear but 4th. My truck had a throttle cable with a knob on the dashboard which I replaced with a push-pull switch and the original knob to activate the overdrive.
