Thanks so much for the advice everyone! My dad and I spent some time today trying the propane torch and PB Blaster, but I still couldn't get it to loosen up...it seems like it is welded in place. Probably not unusual for an older vehicle with high mileage though. The surprising thing was how easy the manifold side nut came off...no problems there.

I am glad to see there are some options besides a full replacement. Looks like the next step will be to just make a clean cut on the pipe and continue with the head job. If anyone else out there has advice I would love to hear it!