I honestly don't think Long was being too realistic when he came up with his plan for 1936 and subsequently 1940. Roosevelt was extremely popular, and even with one of the three people Long was looking into getting to run for '36 (Montana Senator Burton Wheeler, Idaho Senator William Borah, Minnesota Governor Floyd Olson), I doubt he'd be able to even get a dent into the Democratic vote count. Maybe his organization ensures that Louisiana goes to the SOW/Farmer-Labor ticket, but I really doubt Long would be able to cause Roosevelt's loss to the uncharismatic nobody that was Alf Landon.
If Long lives, he would probably need to focus on trying to take out Roosevelt in 1940, where he might of had a chance. Let's say he gathers discontented southern Democrats and some Midwestern progressives with him and at the convention, forces a vote against the then absent Roosevelt. Long's movement would probably be quite progressive and also quite strongly isolationist, and I wouldn't doubt that it would get a lot of delegates lined up behind it. The question is, though, does Long get the nomination?
I have to lean towards no, but only because by 1940, the party structure was nearly one-hundred percent controlled by Roosevelt's New Dealers, as a result of Jim Farley's patronage dishing within the party. Then again, if Long can get a good number of high profile Democrats on his side (Burton Wheeler, Jim Farley (he and Roosevelt weren't on speaking terms, after all), and maybe even John Garner (probably only on the isolationist issue)), he might be able to pull it off.
What happens from there? Presidential nominee Long chooses a western or Midwestern progressive populist to balance the ticket, or, he chooses a conservative Northerner. If its the former, bet on Burton Wheeler. If its the latter, I'm not exactly sure.
If Long is the Democratic nominee, I'm not exactly sure as to what goes on in the General Election. Long versus Willkie is going to be a very interesting Presidential race.