Also, an extremely small number of players have won multiple Smythes (Roy, Gretzky, Orr, Parent, Lemieux, Crosby — am I missing anyone?). So how many retro Smythes does Howe get? Depends on which list you look at, but the consensus seems to be 2-3 (I’ve seen a few lists that give him 4). Given that the other members of the Big 4 all have 2 Smythes and playoff excellence plays a significant role in player legacy, I think this matters. Maybe Hockey Outsider could provide a best guess number of Retro-Smythes for Howe?
It's possible he only wins one. 1955 is obvious - Detroit wins the Cup, and he leads the playoffs in goals and points (setting the all-time record in the latter category).
1949 - probably not. Howe leads the playoffs in goals and points as a 20 year old, but Detroit gets swept in the SC Finals. Two-way force Ted Kennedy likely gets it for the dominant Leafs.
1952 - probably not. In this incredibly low-scoring season, Howe ties for the playoff lead in scoring (but it's an unimpressive total, and it's a four-way tie for first). You'd have to think Sawchuk (5 goals against in a perfect eight game sweep) wins it, especially since star defenseman Red Kelly misses a few games.
1954 - possibly. Canadiens dominate the scoring, but Howe is the highest-scoring non Hab. In game 7 of the first round, Howe scores two goals and assists on the OT winner. But his numbers are unimpressive in the SCF.
1961 - probably not. Howe, in a losing cause, ties for the playoff scoring lead with Pierre Pilote. You'd have to think Pilote (as a defenseman on the winning team) gets the Smythe. (Howe he lead the SC Finals in scoring - but Pilote tied him there as well).
1963 - probably not. Howe leads the playoffs in scoring in a losing cause. He has a decent argument because he leads the SCF in scoring (again). But I suspect Bower or (less deservedly) Keon would have gotten it.
1964 - possibly. This is the 4th time in his career he leads the playoffs in scoring in a losing cause (and the 3rd time he leads the SC Finals in scoring in a losing cause - in a span of four years!) He has a good chance here because it's a close finals series (7 games) and no obvious candidate.
It's possible that Howe retires with a single Conn Smythe (had it existed during his career). But it's also possibly he wins two or three. He almost certainly takes the trophy in 1955, and then there's a bunch of close calls where he's top two or three. The biggest thing that hurts him is four of the six times he led the playoffs in scoring were in losing causes - which makes it an uphill battle.