pin and axle should be quite snug, no tighter than tap it in with only a whack or two with a rubber mallet snug, definitely not slide on through with their own weight.
The bushings will need to be installed in the spindles and then reamed to fit the king pins, light press fit with your hand.
If the largest available oversize kingpin (only one oversize option?) is a loose fit in your axle you should look for a better axle beam. If the oversize pin won't fit you can ream the axle beam to fit the pin, that's standard practice when correcting a worn original to use oversize pins.
