I just ordered some original/used manuals.
A small 1951 GMC operators manual that should have a wiring diagram in it though not sure how detailed, hopefully fully.
Also a 1949 GMC maintenance manual, I can then at least compare the 49 diagram to the reprinted 53/54 diagram, and if no difference then likely 51 is also no difference.

In time I'll find a 51 maintenance manual at a better price. I do like old books and manuals...

•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-