[1project2many]

These are great pictures. Thank you.

1/2 ton trucks use a floating king pin bushing while the 3/4 ton trucks use a pressed in bushing. I haven't found bushing OD specs but it definitely appears the 1/2 ton spindle has a larger hole than the 3/4 ton spindle. Is that a 1/2 ton kingpin bushing I'm seeing in one of the photos? The one with the groove around the OD? Maybe you could measure the OD?

GM lists the 3/4 ton kingpin at .921" diameter while the 1/2 ton is .866." I definitely think a metal sleeve could be used to take up the space between the OD of a 3/4 ton bushing and the ID of the 1/2 ton spindle. Or maybe it will turn out that the spindles are closer in diameter than they appear?

Do you have a plan for the disc conversion once the spindles are installed? The 88-99 GMT400 trucks use a "hat" style rotor. The ID of the rotor pilot is approximately 3.1" while the pilot on the '57 3100 hub is approximately 3.5" OD. I would think a guy with a brake lathe could set up the hubs and turn down the pilot to fit the later rotors. If your master cylinder has the capacity you could match a '98 K1500 rotor up to a 98 K2500 caliper to get greatly improved disc braking. The big trick is making caliper brackets.

Good luck!