Jon,
I doubt I can add much to what you've already thought of.
Is it possible to run the engine so as to get it properly hot?
Could you remove the other studs so that the manifold could be rotated around the seized stud? (if nothing else it might create a small gap for a saw blade or levers).
Can you pry the other end of the manifold away from the block enough to insert a fulcrum of some kind (screwdriver...) between manifold and block, then whop the opposite end back against the block? i.e. use the leverage of the manifold itself.
Grip it with a stud extractor (I mean this kind of thing https://www.lasertools.co.uk/product/0296) and see what gives first?
I'd be frightened of doing a mischief to the block...
Failing that I suggest you need to set up a drilling guide - saw the stud off flush with the manifold, locate a guide on the next two studs, use it to centre a drill while you drill out only to the depth of the manifold, which ought to liberate it; you can then work on removing the remains from the block (a similar approach, but on the bench perhaps).
I doubt I can add much to what you've already thought of.
Is it possible to run the engine so as to get it properly hot?
Could you remove the other studs so that the manifold could be rotated around the seized stud? (if nothing else it might create a small gap for a saw blade or levers).
Can you pry the other end of the manifold away from the block enough to insert a fulcrum of some kind (screwdriver...) between manifold and block, then whop the opposite end back against the block? i.e. use the leverage of the manifold itself.
Grip it with a stud extractor (I mean this kind of thing https://www.lasertools.co.uk/product/0296) and see what gives first?
I'd be frightened of doing a mischief to the block...
Failing that I suggest you need to set up a drilling guide - saw the stud off flush with the manifold, locate a guide on the next two studs, use it to centre a drill while you drill out only to the depth of the manifold, which ought to liberate it; you can then work on removing the remains from the block (a similar approach, but on the bench perhaps).