That's what I figured, plus I wouldn't want to grind away at the pipe.
So True Value has a 1/4 NPT plug for 79 cents (

) and all the guy could tell me is that it's "black pipe". He was actually a real asshole over the phone to me. I asked him what the thread pitch on it was and he said, "I dunno. I don't know what thead you're using." I said, "Well, it's NPT right?" and he said it was so as long as "black pipe" metal is okay, I'll grab it on the way home and FINALLY plug my bung!