I went through this about a year ago.
If you're not removing the gearbox you're in for a fight. This is a good place for an impact wrench. Having rounded off the head it seems a bit late for that. Maybe someone knows of a way that can still be done but I'm not sure how it might work.
If you are removing the gearbox wait until it's out and on the ground or workbench or whatever and start them out with a hammer and sharp chisel. This was my approach since I was removing the gearbox anyway. You might try this with the box still in place but there's not much room to work. Particularly around the upper plug. If you do this be careful not to damage the case.
I realize this isn't very encouraging. Hopefully someone will have a better solution than mine.
One thing I know is that rust is not the problem. I say this in anticipation of those who will tell you to use PB'laster or the like. It won't help in this situation. Maybe someone who understands properties of metals can explain the bond that occurs between the steel plug and aluminum case. All I know is when I finally got the plugs out the threads were clean as a whistle.
One thing I can tell you, if it's not already obvious, is when it's all over and you're replacing the plugs, use anti seize paste.
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'80 DL 2 door, '89 DL Wagon