@Slednut Please post more about your awesome hoist mod and tips for building if you had to do it again and updates, thoughts on your current designs.
Wow this is an old thread, the last time I used the pneumatic tires on the hoist was in 2020 to bring parts of a snowmobile trailer into the shop. I don't have any new pictures and today the hoist has the casters on it. The parts I used are really crude and I would make it more refined if I was to do it over again.
I simply drilled 5/8" holes in the front cross member and welded 5/8" threaded rod to some 1x2" C channel. I cut the C channel so it has a flat spot for the tie rod. The threaded rod goes through the cross member with a nyloc nut on the bottom which acts as the kingpin. I also used the 5/8" threaded rod for the axles.
For the rear I also drilled 5/8" holes through the hoist legs parallel with the front cross member. I also used the 5/8" threaded rod for the rear axles.
One thing I did to make the transition back to the casters easier is weld all the caster nuts to the frame of the hoist so I don't have to fiddle with them.
BTW here's a picture of the engine (351c) that was on the hoist in the original post.