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...the best penetrant, by far, is a 50/50 mix of acetone and ATF. Buy the ATF at the auto parts store, and the acetone in the paint department at Home Depot or Lowes. I tell people this, and they smile and nod at the old guy. But the ones who try it are usually pretty impressed/

That said, I don't think a heat gun will get it hot enough. If it's been together without service for long enough, sometimes you just gotta drill.

Since you have the non drive off, there's a trick where you can pull the rest of the crank by removing the whole spindle, drive side cup, and drive side crank arm with chainwheels as a unit by carefully working out the drive side cup. This makes the whole operation a whole lot easier if you can manage it.

Good luck. I think I stopped working on cottered stuff for exactly this reason.