Other option is to fit one of these manifolds as in photo,drill and tap 4 2BA or 5mm fixing holes.
First while you have 1/4" of the stud extractor sticking out you have nothing to loose by trying to tap it up down left and right with a brass drift or similar to see if it will come out.

Location: Channel Islands

Ian - I'm in North Herts, near Baldock and have a Ruby.If you get stuck and need help you are welcome to make contact.

Location: North Herts

Thanks everyone for the help. I didn't expect such a brilliant response. It was worth buying an Austin just for the this.

Location: Bedfordshire

Ian when you drill out the stud be careful not to drill to far or you can go in to the bore!

Location: Dorset

Collect all the screw/wedge stud extractors in your workshop and put them in the trash bin. They seldom work and cause more disasters than they ever cure. (I am not referring to the extractors which grip the shank of the stud. An excellent product)
Vince Leek

Absolutely!
Personally, the only extractors I've ever had any luck at all with in a long association with broken things are left hand drill bits for the 'almost loose' ones. Or the expensive splined extractors in the Snap-On/Bluepoint kit which comes with drills and guide bushes work sometimes. Otherwise its accurate centre punching and sequential size drilling, (for the flush/sub surface ones) or welding a clearance drilled nut on the above surface ones. Bad language helps...

Location: Far West of New Zealand

The extractors are really only intended for removing studs which have broken due tensio nor fatigue with the threaded part not bottomed or rusted but just awkward to grip. As with all the others, learned the hard way.
The difference between a bodge and an ingenious repair is often slight. Instead of the original bolt, By using a stud with spanner flats on the projecting part it can be held from revolving enabling a variety of effective remedies.

Location: Auckland, NZ

Don't forget to wear eye protection when using the darned things. A fellow apprentice lost an eye when he shattered a tap.
Dave.

Location: Sheffield

I have had success grinding down stud extractors using Diamond Tile drill bits similar to the ones advertised here. http://www.plumbase.co.uk/rothenberger-diamond-tile-5203-2205226

While they don't have the ability to self center, I have been able to grind the extractor and bolt out close to the threads then re-tap the original thread to clean out the rest of the old bolt.

Pulling the block off is not overly easy and is also inviting the proverbial can of worms to the party. suddenly a broken stud turns into more broken studs, a re-bore or rebuild depending on what you see when you lift the block.

Location: Sunshine Coast Queensland with Skippy

Oh dear this is a problem that as you now have realised should not of happened.
Most of us have done similar and like you learnt the hard way.

Difficult to say without seeing the size of extractor you have got in there but I would suggest removing the offending centre part of the bridge by carefully cut grinding back at a 45 degree angle at each end of the bridge.
Then make a plate designed to mate into the space (with hole and thread in the right place for the stud) this trimmed at 45 degrees opposite way to the trim in the block. This should be vey slightly (10 thou or so) below the flange surface when in position.
Attach with suitable glue simply to hold temporally.
Refit the manifold straight away before glue sets.
Have done this many times when the centre is too badly corroded to use.
You do not need to remove the engine or block to do this.
Careful and slow is the way to go.
Hope this might give you a help.

Location: The Centre of the Universe

Ian.

I will mention it as no one else has.

I would say that if you feel you cannot carry out the repairs in situ. My belief is that it is easier to take the engine out rather than take the block off and refit in situ.

Location: not north wales any more

I agree with Hedd. Easier with engine out. If any one wants to borrow my stud extractors, they'll need a wetsuit. I live next to a canal.

Location: Wales

I don't think I'd be brave enough to try this with an assembled engine, and you would need the engine out of the car, but if you have access to a TIG welder...

Stud Extraction using TIG