First a note on the service manual, a most invaluable tool for these old trucks.
The chevy manual is easily found online thanks to Keith Hardy. .
http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htm
The Chevy manual is a good reference even for the GMC because so much is the same between them.

I do like to have the right manual and found a used reprint of the correct GMC manual, you can buy a new one here from Jim Carter

I'm very impressed with the GMC manual, it goes into much more detail and covers more stuff than the Chevy manual does. For example recently there was a question about lubricating the brake cross shaft where it passes into the frame. The Chevy manual does not mention this in the lubrication section of the manual or in the brake section. The GMC manual does clearly explain in the lubrication section that it should be oiled every 1,000 miles with a squirt can of engine oil. Also noticed more detail on brake servicing, and I'm sure more good stuff throughout the GMC manual.
I recommend anyone with a Chevy should also get the GMC manual, I wish I had 16 years ago!

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-