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When new, those kingpins were a very light drive fit in the axle, unfortunately after decades of rust they can be very hard to remove. You need an acetylene torch or at least a map gas torch, propane usually won't cut it. Put some blocking under the axle at the spring hanger so it is solid, heat only the end of the axle red hot, do this as quickly as possible so you don't get the kingpin to hot, give it a good hard whack with a 3lb. hammer. Good luck, hopefully you won't have to remove the axle and get it in a press.
What he said ^^^ and the heat n beat... I've even used the freeze spray also, not sure if it helped, and acetone/atf mix or PB blaster, don't think I've ever had an "easy" king pin rplc'ment, in fact, I have a new set in my parts tool box for my 66 that I have put off doing for far too long... and YES on the MAP gas at minimum, might even try a heat cycle or 3 if no-go the first time, with a soak of the aforementioned mixes in between.
There’s a product from Laco called Cool Gel. Comes in a spray bottle and will keep surrounding parts from overheating. It’s a plumbing/pipefitting product and sold at supply houses.