I believe your're off a little on the dimensions, these rear spring and shackle pin lock bolts are same for 3/4 ton through 2 ton AD trucks.
The parts book shows them as 5/16-24 thread, which sounds familiar, and all but 2" long.
http://chevy.oldcarmanualproject.com/parts/1929_54/438.htm
Part number 3656671
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•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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