I'm almost entirely certain that not all of the bosses in Naxx are Lvl 63.

However, I'd like [if possible] to have a list of the ones that are.

I'm 99% sure that at least Patchwerk, Thaddius, Faerlina, and Loatheb are.

Is there any definitive data on this that someone could expunge?

I'm not sure why any boss would be anything BUT level 63 since altering the level difference even just one is huge. Perhaps what you're referring to is weapon skill of certain bosses, since Patchwerk can't crush your tank? Level 64 would cause 55% glancing blows, and I think that would have been noticed and parsed by now. Level 62 probably wouldn't be as noticeable, but what would be the point of lowering a boss's level?

Every boss in Naxx (and every other raid zone, I believe) is 63. If a boss was level 62 or 64, there would be a huge, gigantic difference in resist/miss rates.

Some bosses have their skill/attack level whatever lowered to 310, so that they can't crushing blow. Patchwerk is the biggest example of this. He's designed as a pure healing/damage-in spreadsheet.