Forgotten.... What thread size are head studs into the block ?

[Lynn Eades]

Richard, Try not to use a tap, unless you can get a well worn one.
Try grinding a groove in an old stud with your angle grinder, and use that. The least amount of "meat" you cut out, the better. It will be important to use a good thread seal anyhow, to prevent water running up the head studs.
As you know a slight relief will also help to transfer "load" out into the deck, as long as you are careful as the new studs will have a "bulge where they seat into the block deck.