A faster ratio rear axle would be a good start, probably simplest and cheapest option.
The OD Clark transmissions are not easy to get parts for now, some parts not available new, I would stay away from those unless you find a good one at a great price.
What type of clutch housing/bell housing do you have? is it a round SAE style? If so then you can choose from a number of more modern OD transmissions.

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-