3800 is a 1 ton, and should use same king pin as 3/4 ton. (From memory 0.9something" kingpin diameter)
4400 or 4100 may also have same axle beam as above but uses 5 lug hubs, same King pin as above. (Exception listed below)
6100 and 6400 have heavier axle, 1.109" kingpin, and was a heavy duty option on the 4000 series 1.5 ton.
Napa is an easy source for all king pin kits in question.
