I have a trick with an acetylene torch that can also be done with a regular propane torch you get at the hardware store. It just takes longer. Heat the head of the through bolt and as much of the threaded end as you can. With acetylene, get the ends orange hot. You don't want yellow, as that's when the steel is starting to liquify. A little propane torch won't get it that hot, so if you can even see a hint of red glow in the bolt head, that's good enough. Otherwise, just expect to heat soak the bolt for a few minutes at a time. Allow the ends to cool a bit and repeat a time or three as needed until the bolt starts turning by itself. (I prefer an impact to break them loose, but the bolt itself could be eroded to the point that it'll snap no matter what.) The big idea with the heat is that you're sending the heat down the length of the bolt and expanding it to the point that it breaks up the rust that's binding it. I can't say it'll work for your rig. 20 years and 300K in prime rust belt country might win this one. One other caveat to heed is that after all this time, the sleeve that's bonded to the bushing itself might already be broken away and will spin with the bolt it's seized to. Make sure you have all the parts you need for this one just in case. It's decidedly plausible you're going to have to destroy old parts, one way or the next.