Welcome to VCCA Chat. That sounds like great original truck.
As you know, changing the rear axle ratio will change the relationship between engine speed and ground speed. In your case, I am assuming you want to reduce the engine speed for a given ground speed.
I’m not an expert on your truck so I do not know what other axle ratios might be available. Typically for that old a 1 1/2 ton truck there was only one ratio and it was intended to give pulling power, not speed. The other factor to consider is that even if you could install a higher speed ratio the engine might not have the power to make the vehicle go any faster. Plus your acceleration will be reduced.
A key point which you note is that brakes are a limitation. Those mechanical brakes need to be properly maintained and adjusted. As a first step I suggest you get a shop manual and go through the adjustment procedure. You might be surprised how much better they will work.
That whole truck was never designed for nor intended to be driven at today’s highway speeds. If that is your objective you probably need to consider putting the body on a more modern truck frame and chassis. The steering, brakes, frame, suspension, and wheels/tires were designed for use in an era with lots of unpacked roads and speeds of 35 mph.
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