All good points.
Just a .75 OD ratio will only gain about 15 mph, not going to be real speedy even if it had plenty of power. And otherwise with the first 4 gears will drive just as it does know. The OD would be handy on moderate down hill and level, but if it struggled to pull any given hill in high gear now there's no way you could do it in OD either.

Calculate speed here http://www.onlineconversion.com/bigger_tires.htm

•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-