Removing stuck shift knob?

So I'm currently trying to remove the shift knob from my 1976 A833OD so I can refinish it and make it all pretty again, and it's like, super stuck. I can see that the whole nut insert is covered in rust and so are the threads. I've let it sit in WD-40 and PB blaster for a little bit (not like overnight), and I still can't get it to come off. I really, really want to restore this shift knob and not have to get a replacement one (unless someone on here has a NOS mopar part #4019132 or 3575480 for cheap), so destructive removal methods aren't in the cards.

My main question is, can I put some DX579 (phosphoric acid) on the threads without melting the plastic, or should I try heating up just the shifter handle with a torch and then try to remove it? Any help is greatly appreciated.