Be aware that if you gear up very much the engine might not be up to the task, torque-wise. There's a reason the truck has a slow/slower option with that 2 speed rear. The old 216's and splasher 235's didn't have a great deal of torque, and raising the gear ratio will have you downshifting so often you'll feel like you're rowing it with the shift lever. The SM-420 with a Brownie auxiliary trans might be the best option, which will give you a selection of under/direct/overdrive ratios for every gear in the main box. It turns your 4 speed into a 12 speed overdrive setup. On the rare occasion you'll be able to get into top gear, you'll have good road speed. Don't forget to do a major upgrade of the brakes and do a total overhaul of the steering- - - -getting it up to speed is only half the job, and keeping it between the ditches and getting stopped from higher speeds can get exciting.
Jerry
