Drilled and tapped my first oil gallery plug. This is the 5mm plug that was pulled to extend the oil gallery. A 4.5 mm wrench fit the tap perfectly so I used a combination of that and a lot of patience...
Some notes:
1) The oil gallery itself almost perfectly fit a 15/64 inch twist bit and not a 1/4 inch. This coincidentally matches in an "A size" bit.
2) A 1/16 NPT tap calls for a pilot hole made by a "D size" bit. I only had 1/4 inch available and decided to use that to a depth of roughly half the pipe plug length.YMMV.
3) An NPT fitting calls for between 1 and 2 turns past finger tight upon installation to seal (in conjunction with some type of thread stealant, my preference being Teflon paste). This plug ended up just about as deep as I wanted to see and has about 1.5 threads still showing at finger tight.