I am of the impression that a third party candidacy would not have come from the liberal wing of the Democratic Party but from the "liberal" wing of the Republican Party.
I kind of think that the perfect third party candidate in this scenario is Jacob Javits.
Nelson Rockefeller, the sitting VP, may have run a 3rd party bid if it became clear early that Ford was going to be dumped by Reagan.
HHH knew he had cancer in 1976. Javits may well have know he had ALS by then. There was Lowell Weicker and Edward Brooke.