Great, that's a start.
What is the OD ratio for your current transmission?
From what I can find a 700R4 has an OD ratio of 0.70 to 1, is this right?

If you were to run the 700R4 with your current rear and tires you would turn 1,840 rpm at 65 mph.
do you have an idea of what rpm you want to turn at what road speed?

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-