According to my book, C6ME could be a 360, 390, 410, 428, AND various truck engines. Sorry, but no 427 listed for C6ME. KEep your money in your pocket. Really, the only visual clue would be to look in one of the center freeze plug holes for the correct engine size stamp...and then I am not sure they did that for the 427. Other clues might be the cross bolts and so on. As for the freeze plug test, my guess is he won't let you do that.
Mike