bolt grip extractors,

run the engine to warm the oil

warm the female threads of the tack welded nut further with one or two soldering irons held against it on either side,

but chill the rounded off bolt head with either a chunk of dry ice, or this

http://www.amazon.com/CRC-05002-Freeze-Off-Super-Penetrant/dp/B000TFTH00

if you cant locally find the Freeze off spray
or dry ice ( call supermarkets or welding supplies)
then go to the drugstores and get the wart freezing kits

if any friends work at science departments at universities ask if they can bring you a thermos jug of liquid nitrogen

hot female threads + chilled male threads + grip extractor sockets