I've just been through this also. I took my 51 3100 to a shop that mainly repairs straight axle big trucks, and the old guy who owns the shop told me that usually it's the axle that wears out instead of the spindles, which I assumed were shot. They had to bore out and resleeve the axle, and was able to use the spindles as they were.
I would stay with the stock king pin size instead of oversizing, oversize king pins aren't as easy to find.
