I think I used 3/8" and I don't remember if I drilled it or not. I seem to remember the holes being pretty decent sized... Drill bit for a 3/8" NPT tap is 9/16"
Don't worry about messing it up.. If you are worried about it I can help. Even if the holes are too big the tapered tap works great and you can get some decent threads in there. I used anaerobic sealant on them also.
Nice thing about tapping it also was that I was able to use the elbows which kept it really low profile.
Here is my thread on it:
looks like i did use 3/8" pipe threads in it. there are a few pictures in the thread.