When you remove the head bolts you need to clean out any and all oil in the hole and any debris which may have collected there; Failure to do so may prevent the allen bit from entering the opening completely.
If you cannot get a bolt out all the way using constant steady pressure AND the socket is in all the way you can try: Pounding downward on the head of the bolt a few times with a rubber mallet; Use a breaker bar and while you hold it FIRMLY in place have someone tap the end of the handle several timed with a hammer; AND, spray the area thoroughly with a GOOD penetrating oil (WD-40 does not count as a good penetrant).
As for the bolts you've already stripped, go to the hardware store and try to find an allen bit or regular allen wrench that's just a *tiny* bit larger than yours, it'll most likely be an SAE size unless you've got a shop in the area which can order .5mm bits. Take it home and tap it in with a hammer... you know the drill from there. Some people have luck with easy outs; Grinding the head off the bolt usually results in some heavy gouges in the mating area directly under the bolt head and as such getting an accurate torque thereafter is unlikely without having the area machined.... not to mention all the debris you'll have to worry about flushing out of the block. Note: If all you can find that'll fit is a regular L shaped allen wrench you can just cut it off and slip it into a regular socket.
If I think of anything else I'll let you know.
EDIT: Oh, and don't be afraid to use a LOT of penetrant... it's cheap enough. Soak the bolts, wait an hour, soak again, wait another hour, soak again.... repeat a often as you wish, then let it sit for a few hours before going at it again. When you do go at it again make sure the allen is going in all the way before you try to remove the bolt.