In '76, McGovern has a shot against either Ford (assuming Nixon was re-elected in his second race against Humphrey and still picks ford and resigns like IOTL) or Reagan (who may be able to beat Ford with just a few butterflies, let along a whole four years of them). Reagan would probably be the better competitor in a lot of ways because it'd be much harder to paint your opponent as a dangerous radical when they can call
you a dangerous radical with maybe about as much credibility.
Both Reagan and McGovern basically have to pick moderate running mates. I think Reagan might aim more directly for the South in this election and pick somebody like Howard Baker, Bill Brock, or maybe John Connally while McGovern will either want a guy unions like (like Eagleton in '72 IOTL) to shore up his support there. Just from the start, Reagan is probably going to win California, most of the South, and much of the Republican bastions in the West. My preferred pick with Reagan would be Baker, but I have a feeling that if he could pick anybody he wanted with his eyes on the South, it'd be Bill Brock.
McGovern's best picks are Ted Kennedy, Reuben Askew, Terry Sanford, Edmund Muskie, or Abraham Ribicoff. I doubt Ted Kennedy could be convinced to accept it unless he's really scared of Reagan winning. Askew or Sanford would be a great choice as both are liberal Southerners who couldn't win him Dixie but could help tip the national scales more in McGovern's favor. Muskie and Ribicoff could be convinced, and could be effective, but neither would have much to gain from the run. Muskie ran with Humphrey back in '68 and probably wouldn't be thrilled about having his name attached to two losing tickets. Ribicoff was a McGovern ally who nominated him at the '68 Convention after Robert Kennedy had been killed, but Ribicoff had already spent 15 years in D.C. and had turned down McGovern's IOTL offer to be VP. I think, for a few reasons, Reuben Askew would be the one to rise to the top. Askew was known as just a great person and I think that's something McGovern would latch onto and appreciate from a VP the most. On top of him being a Southerner and seeing eye-to-eye with the party's progressives and the establishment.
George McGovern/Reuben Askew vs. Ronald Reagan/Bill Brock would be quite a race in '76. I think after 8 years of Nixon then Ford, only for the incumbent Ford to be taken down by a Goldwaterite movie star, it'd be more of an uphill battle for Reagan than anybody might assume just looking at who were winners and losers in real life.
McGovern ran as the very left wing candidate in '72 and just barely lost. Coming back around, he would probably come across as more polished and mainstream. He entered the race as the frontrunner for the nomination this time around, rather than as the untested radical. McGovern is more of a household name and easily recognizable among Democrats as the man who just barely lost to Humphrey. The US has had some big changes since McGovern's first run was ridiculed as being in favor of "amnesty, abortion, and acid," but the war's over ('75), abortion is legal ('73), and the man who declared the war on drugs had to resign in disgrace ('74). McGovern doesn't seem quite as radical now as he may have four years before. Not to bring this into modern politics, but there'd be a little bit of Bernie Sanders right now in the public image of McGovern. In other ways, McGovern would also be playing the part of Reagan in 1980. The country has come around a little more to seeing things like George McGovern does and that would really be apparent.
Ronald Reagan, on the other hand, is this race's radical. Anti-abortion, pro-military build up, and only 12 years out from Barry Goldwater's run, Reagan will be a harder sell to the American people than he was in 1980. Despite sharing a home state with Nixon, neither of them are that tied together, which has its upsides and downsides. The upside is that Reagan wouldn't be carrying the weight of Watergate like how Ford was, but the party's name is still tarnished. Bringing another Barry Goldwater forward right after one of the worst corruption scandals ever just blew up in the GOP's White House isn't much of a winning strategy. Sure, Reagan has charisma, but up against somebody like McGovern, who I would call a downright intellectual, he could seem broad and a little shallow as a thinker and a statesman. McGovern spoke eloquently on topics and had a calming demeanor of him that could be a really refreshing change of pace after Nixon (who everybody saw blow up as his presidency unraveled) and Ford (who had a habit for being clumsy). Overall, it's easy to pick apart things on a macro-scale as inevitabilities, but I don't believe the conservative movement winning over politics was inevitable, nor do I believe that an OTL winner will always be a winner and an OTL loser will always be a loser.
EDIT:
Given how narrowly Carter defeated Ford, I think McGovern would lose if nominated. Sure, Ford was unpopular in 1976. But unlike Carter, McGovern could easily be painted as too liberal. And if his 1972 race shows for anything, McGovern had awful political instincts. Ford wouldn't win in a 49 state blowout like Nixon, but he'd beat McGovern.
I know I made my what if based off of him facing Reagan, but I don't think Ford would do as well as he did against Carter, not by a long shot. Jimmy Carter suffered from lack of name recognition and generally being a bad campaigner who was far more inept than McGovern. McGovern was more of an idealist who usually stuck to his gut and did what he thought was right rather than popular. You wouldn't catch him on the trail saying he had lust in his heart in a Playboy Magazine interview.
If it's McGovern vs. Ford, I think it's going to be a photo-finish, whereas McGovern vs. Reagan would be close but less so, with the results being a little lopsided one way or the other.