Well, actually, I had the engine out while doing the gearbox, but I got the four broken head studs out and replaced without removing the cylinder heads – just the tappet covers off.
With the engine back in the car, and looking at spacing around the tappet covers, this method to extract and replace broken head studs should work with the engine still in the car. Not so comfortable, but likely do-able.
I made my extraction tool from an 8mm Koken nut twister, welded to a ¼ inch socket extension and turned down to just over 10mm outside diameter. I also had Plan B and Plan C, but didn’t need to use them. All described here:
https://www.youtube.com/watch?v=KQyXD_deDWA
Hopefully this saves some heartache for some of you!
Cheers, Brian