I too have heard that the 4BT engines shake really bad.
However, I don't think there is anything really wrong with them, but there isn't anything unique or spectacular either.

That said, I still think that for most people the Cummins 4BT is probably the best choice for a diesel swap in an old pickup. Although if there is room then the 6BT would be better for a real work truck.

My personal choice for a unique and capable diesel swap is the Detroit Diesel 4-53T.

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-