The lower pivot shaft is inside a hardened steel spacer which rotates in the bronze bushings. High carbon steel rusts more, and faster, than mild steel. There's no easy way to grease the bushings, shaft and spacer without disassembly, so no one does this. Thus condensation and rain cause the shaft & spacer to rust-weld themselves together over time. If you don't want to cut the ends off the steel a-arm, then reweld them, the solution is to cut the lower pivot shaft ends off just inside the a-arm ends. Cut right thru both of the upright's steel end caps without touching the forged steel upright.
This used to be so common that Hall Pantera once sold a 'repair kit' that consisted of a new lower shaft, two end caps, the thrust washers inside, a new spacer and a pair of new bronze bushings. Once the ends are cut off, the a-arm can be removed undamaged and the cut shaft can be knocked out of the hopefully undamaged upright. Once out, you'll be amazed how much force it will take to press the two ruined parts apart when you have full access and can apply heat where its needed. 12 tons of press force may not be enough....
Those are your two choices, unless you get very lucky with the press and soaking over days/weeks. Heat likely will do nothing since you cannot reach the trouble area. After cutting the pivot shaft, if you own a metal lathe, you might consider making a custom inner spacer from 321 Stainless in place of the stock high carbon steel part, so condensation cannot ever again rust the parts in place. There are also various schemes in the archives to modify things to grease the insides without disassembly, but that does you no good now. Pantera specialty shops will fix this for you in exactly the way I described. It will not be cheap but you won't need to worry about wrecking your upright with a Sawzall, either. Good luck.