First, what rear ratio, and tire size do you have?
How fast do you want to go at what engine RPM?

My first thought for a cheap overdrive is to swap the transmission for a Spicer 3053A from a military 2.5 ton. That will give you 0.80 OD. It has a round SAE clutch housing that is #2, but they come with an adapter ring for the larger #1. You probably have a #2 flywheel housing on your engine?

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-