Likely the kingpins are OK, not worn. Bushings might be easier,or as easy, to just replace while you have it apart.

If you haven't thought of how to replace bushings I recommend fixing them in the spindles on the 1/2 tons, instructions here:
Kingpin Instructions

What did you torque the spindle nut to? Only 33ft lb prior to backing it off for proper wheel bearing adjustment?
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0307.htm

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-