I did it in the last millennium on my '57 1/2 ton GMC. No big deal. Assuming the king pin to axle interface is tight (it should be), I'd replace the king pins if they measure out of spec (0.866"-0.8665"), the bushings for sure, the thrust washer parts, the zerks, the expansion plugs and maybe the draw keys. Service the wheel bearings and maybe replace the wheel bearing grease seals while you have them apart. I just checked the specs and the drawings of '57 and '51. Both are the same, including dimensions, so I assume your 55.1 is the same. A cross section drawing around the king pin can be found here. 1955 - 1959 GMC Trucks Maintenance Manual - Models 100 - 500 By clicking the "previous page" and "next page" at the bottom of the page you can get more info on doing this job. Specs are on page 15. The drawing above shows ball bearings as a thrust bearing. I've never seen one like this in the real world. I've only seen the ones with thrust washers as shown here:
Update: Our host has different sets for 47-54 and 55-59. I don't know where your 1st series 55 lands.
Found these two things on the Stovebolt page. https://www.stovebolt.com/techtips/kingpin.pdf Kingpin Replacement for your old truck
