I suggest two teams.
First, the 39-40 Bruins.
This team didn't have the late '70ies Habs record, but they came up with a relatively decent .698 PTS%. But it was a 7-teams league, with six making the playoffs.
That season, the Habs were... unspeakably awful and ended up with a .260PTS%.
However, seven of the Habs wins (out of 10) came before January.
The situation is eerily similar in 40-41, this time with the Americans. Six wins in before january, and 8 wins total, but more ties ended up giving them a better record than the 39-40 Habs.
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Other candidate : 44-45 Habs. Two teams missing the playoffs (out of six), the Rangers, the Hawks and the Bruins are all very bad and end up between .320 and .360. Habs ended up with .800 (and didn't lose a game in two months somewhere between JAN. and MAR.). For all we know, the Habs might have already clinched by Jan 31st, at which point they had 17 games remaining (a whopping third of the season), and had a record of 26-5-2.
Had the Bruins won ALL their games starting from Feb 1st, they would have finished 28-21-1. Which places them 3 points ahead of the Habs... And there was two worst teams -- against which the Bruins would play.
On Feb 1st, the Hawks couldn't possibly catch the Habs if wins were the tiebreaker (winning all their games would put them at 24-22-6).
On same Feb 1st, Rangers were 7-19-7. With 17 wins, that gives them 24-19-7. Which beats the Habs by 1 points.
Scenarios with Rangers, Hawks and Bruins winning all their games requires wins against each other.
Conclusion : The 44-45 Habs had clinched by the end of January. Every scenario where team passes the Habs include the other two teams falling behind. It was probably done a few days earlier. Just bothered looking at Jan.31st due to easy cutoff date.