As in the post mentioned/linked above, I mentioned in 2004 that one of the engines I have has had the holes TAPPED for Allen Plugs. I'm only guessing (not sitting here in front of me
) that they were UNC thread, they could have been NPT, but find what plugs you can and tap that thread to suit. There is more than sufficient metal in the threads to tap into, and part of the plugs protrude into the water jacket, but they dont get in the way of anything.
Ian, you wouldn't need to seal the thread so much from leakage, as the sump gasket and some RTV Silicone would cover it, but more so as a corrosion inhibitor/antiseize to allow you to take the plugs out easily as often as you want to clean out the crud in the bottom of your water jackets. I'd guess you'd use the same thread sealer (Loctite, etc) that you would use on those head studs that thread into the water jacket from the top.