The last F&%ing manifold bolt!

green1 said
Cut the head off, pull the manifold; now you can get to the meat.

I agree with this!

Use a dremel if you can't get a 4" cutting wheel in there. Then you'll have a long stub to work with and can concentrate heat right at the source, head/bolt junction, not an inch or two up the bolt.

Weld a nut on the remainder and get after it.