" In your example, the total vote in the Senate was 63 votes. Your 42 was 2/3 of those VOTING, not present."  How do you know who was present?  Were you there?
It is quite common, then as today, for Senators to leave the chamber when a vote comes up that they do not want to vote yes or no on, and go back to their offices for a drink, and come back after the vote.  The vote was 42-21 exactly two-thirds majority.
The fact that they didn't think they had the votes in the Senate is meaningless, as to everyone's surprise it passed by a two-thirds majority.