If you are changing bellhousings (going from stick to auto) you could just sawzall it right through the bellhousing. Leave enough to vise grip out after the trans is removed. That is the destructive way.
Weld a nut to the top of the bolt head works a lot, if you can get in there.
Hammer a socket on is another great trick. Find a socket that is just a hair too small and beat it on. It will cost you $10 to learn how to get the socket off the bolt later;D
Grind flats onto the bolt head to put an open ended wrench on it.
Grind the whole bolt head off.
Easy out, but those tend to break on stuck bolts.
Drilling I would avoid until the trans is removed and you have exhausted normal bolt extraction methods.