forget easy outs, you'll break those too. If it only 2.5MM above the block deck best bet would be to pull the head & weld a new hex head bole onto the broken one then unbolt it. I'd prep the broken bolt by flattening the top if its sheared in as funky way.

There is always vice grips as a 1st attempt prior to welding.

good luck!

OK, read your last post more thoroughly, so you have threads above the block? If so the nut idea isn't bad but I'd consider running it down to the block surface then infilling the remaining exposed inside portion of the nut with a spot / plug weld then welding another bolt on top of it for removal. You'll probably only get one shot at this before its a block trip to the machinist.