I like long threads. It's interesting to read opinions from 3 years ago. The OP compared Mussina to Jimmy Key, and at the time, the comparison wasn't far off. I don't think anyone would look at Key's and Mussina's records as being similar now.
STLCards2 and Classic both hit the nail on the head saying that he wasn't worthy at the time but would probably have enough credentials with a few more good seasons.
I don't think Guidry cuts it. He pitched less than 2500 innings, and his ERA+ is 119. He's sometimes compared to Koufax, but Koufax had an ERA+ of 131, and Koufax's peak was better. Guidry's exceptional record was helped by playing for the Yankees. I think his peak was certainly HOF worthy, and one more 200+ inning season with an ERA under 3.00 probably would have been enough for me.