If the PB Blaster doesn't work, you might also try buying an "impact driver".

It's a device that looks kind of like a counter-punch, but at the end of it has a ratchet-like system that translates some of the pounding force (impact) at the far end into twisting action at the screw-head. Some of the force goes into the head of the screw, and some goes towards twisting, so you don't strip the head (hopefully). A few good whacks (anchor your peiece firmly in a vise perhaps), should provide just enough force to crack the rust-bond loose between threads and hole. You can then finish unscrewing it with a conventional scewdriver.

Good Luck!