I have used a high speed drill such as the Dremel with a diamond
impregnated bit to drill a hole into the bolt, then drive an easy out
into it and twist out.  If that still does not work just destroy the
bolt with the diamond bit.  These diamond tools are now fairly common
and not too expensive.  Made in China.
 Of course it might work OK with just one bolt holding the slave on.
No real experience with that approach.

what darthpup says, but with this variation - use the dremel tool to cut a
slot in the screw and use a screwdriver to extract. AeroKroil is great for
helping unseize bolts, spray with Kroil and wait a few hours - repeat, then
remove.  If it is seized solid, use a left hand drill - first you need to
get a pilot hole so you dont' keep slipping into the aluminum, then drill
out with the LH drill - usually the LH drill will at some point bind and
unscrew the thing for you - the bolt is not hardened.  using a air driven
right angle drill may help.  Be prepared to helicoil the hole if you mess it
up.  Using one bolt will be a disaster waiting to happen, don't do it.  if
you are close to needing a clutch, this is a great time to do the clutch job
and drill out the  bolt while you have it apart (by the way, I have a spare
bell housing if you would like a replacement)