As mentioned, cut the head off, remove cover, evaluate how to get the rest of bolt out.
You can apply heat to the cover to try and get some expansion to break the bond. Tapping the cover with a hammer can help
A propane torch around the area to heat it can work along with wicking in candle wax or PB blaster when hot. Heat up the block, not the bolt. That area of the block is pretty thick and heat won't normally effect the surface. Smacking it with a hammer when heated up will shock the threads/rust and help with removal.
Freezing the bolt is another option. Most people don't have a compressed gas around to do this.
