40 mph, and you are sure you are on the high side of the rear, something sounds fishy.

Is there by chance an extra gear, and you just have not found it? It could be the shift rail for that position is rusted in neutral, so you can move the stick over there, but not move it into gear?

Is the engine running right? The timing set right, plug wires in the right places....

•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-