I had the same problem with a dead cat. I decided to chuck the manifolds and use headers. Would have been done long ago but decided to add cats to the headers and therefore, complications arose.

I think I busted 2 studs, but with enough left to still get 2 nuts on them to get them out. You really want to try and get a wrench on there completely parallel to the block--off angle torque is what shears them. It's not terribly difficult, and if you take your time you should be fine. Good luck.