The 128 heads I have drilled before didn't have a boss for the Fcar bracket. (Have to use sealant on the stud there) ZZ4 heads did, I've also done some 113 (almost the same as ZZ4s) but I can't remember if it had the extra material for the boss.
I can tell you that you can see the extra boss through in the coolant port. If you have the deep boss, bolt on your pass accessory bracket, use a bigger drill bit that just fits in the bracket to locate the center and then give the cyl head a shallow centering mark. Remove bracket, stick correct (for tap) size drill bit in another accessory hole that's close, use tape on bit for the depth, drill and then tap it. Also use a bottom tap to make the threads clean all the way to the bottom.
Takes maybe 10 mins

 

Drilled ZZ4 on left engine, non drilled 128 on right but you can see circle where the bracket hits it.

 

I drilled 1/2" deep 3/8" -16 bottoming tap and had worked good for 2 years did not break into the water jacket.

 

FWIW - I drilled my LB9 head for the serpentine brackets and (as expected) hit the water passage. You just use some thread sealer on it. Just like a head bolt. It's not an issue really. Honestly everything I've read indicates it's expected that the accessory bracket holes may hit water jackets in that location.

GD