I changed it to Superman: The Movie: Extended Cut. It may be a television cut, but it was officially released by Warner Brothers. Richard Donner hasn't seen Superman III (Donner Cut Commentary), so I personally don't believe he dictates canon. Superman: The Movie: Extended Cut isn't much of a regular film in that it really is just a more detailed version of events. It shouldn't be treated as the definitive cut, since the pacing is worse, but if you're going to watch a longer version of the film, you might as well watch the longest.
Plus, it's just a whole lot more consistent to view all Extended Cuts as simply an "extended" version of the film. The only exception might be X-Men: Days of Future Past: The Rogue Cut, which isn't canon per X-Men: Apocalypse's Commentary, but even then you can just about fit it if you wanted to.
A longer version of Superman II and Superman III were made I believe, but they were never made available officially since they aired. I don't see them as canon as Warner Brothers hasn't released them since. I don't think the longer Superman II could even work anyway, since he destroys the Fortress presumably (I'm guessing) to move it so Luthor, the Arctic Police and the Kryptonian Renegades don't find it, which Luthor does in Superman Returns as he remembers finding it before.
If they do anything like a tv show or crossover, I'll move everything to modern day. I don't really like having to put everything in the past because it creates a convoluted reason why this universe is set in the past while also being set in the present. I just feel like a lot more points to the 70s-80s that I can't ignore.
Is it reasonable to assume that the Superman who is wearing the Kingdom Come suit in Paragons Rising is a completely different character, as Donnerverse Superman is completely unaware of the ongoing Crisis, or are we to assume he recognized Clark from the fight with the Luthors, hence the "nice glasses" comment, but blew him off at first for some reason? I think it's pretty obvious that the Luthor who knocked out Lyla is Gene Hackman, though.