I've got a 3/4 ton with the 4 speed tranny and 4.56 gears. I can go 65 mph as a maximum cruise, but I'm spinning the engine at around 3200 rpm. I can hit 70 for short bursts, but it's still barely adequate to keep up with traffic.
Therefore, I have looked into Gear Vendors and found that they do sell an OD unit for the SM-420. You're looking at around 2 grand, but it could give you a true 8 speed transmission, with a supergranny gear, a granny gear, and a 0.78 overdrive in 4th over.
The other option is to adapt a rear end from a later truck. But for a Stovebolt that has limited rpm ability, more gears would be just what the doctor ordered.
By the way, the brakes are strong, but not being power brakes, you have to stand on them pretty hard in a situation, but they do respond. I've even towed with the stock brakes, and towing a car on a flat bed trailer, the brakes will do the job, but you have to plan ahead.
