You might look in the links page here on Stovebolt.Obsolete Chevy has a lot of larger truck parts.Their catalog lists std size, but they often have parts not listed in their catalog.Egge auto parts sometimes has king pin sets.Some other guys here probably have additional sources.
Having the axle king pin eyes resized with a sleeve is an option.Use caution, I actually saw the damage caused by a resleeved axle eye that split causing the wheel and spindle to fly off on the road.