An NV4500 transmission is probably a good swap to gain one faster gear, 0.73 OD (about).
Swapping the rear axle for a single speed from a 1.5 ton may be cheaper and accomplish the same goal?

I've seen plenty of small auxiliary transmissions for sale, and I think $1,000 would be way to much for one even with mounts and a shifter. I sold one without any extras for a whole lot less than that and it took a while to sell.

Calculate gear ratios and speeds here
http://www.onlineconversion.com/bigger_tires.htm

At some point you either don't have enough engine to pull the truck along at the speed you desire, and or you don't have safe enough or large enough brakes to be going that fast in it any how.

In my mind that cutoff point is around 60 mph, and if you want to go faster start with better brakes, a dual circuit MC will be a good start. Then at probably 65-70 you'll run out of HP and just waste fuel trying to go fast.. again, just my guesses on that.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-