I think you'd end up seeing a lot of the same faces from the 1980 primaries running 4 years earlier. Aside from Reagan, John Connally probably gets into the race, as do Baker, Anderson, and Crane. Kemp and Rockefeller probably round out the list. I don't see Bush giving up CIA to run for President in 1976. I don't think Reagan is a shoo-in for the nomination since a lot of his success that year came as the conservative alternative and the outsider running against the establishment Ford. If Rockefeller is the VP, and thus frontrunner, then maybe Reagan would still run a similar campaign. But he's going to be competing against other conservative figures, too, who might suck the oxygen out of that kind of candidacy.
Despite our collective memory of the primaries that year, Ford actually won the early states pretty convincingly. Ultimately, he won the majority of states and took 53% of the popular votes in those races. Those aren't great numbers for an incumbent, but Ford wasn't a typical incumbent. Truthfully, I think Ford's victory, however narrow, points to a moderate Republican winning the nomination rather than an appetite for conservatism with the deciding factor being which side lines up behind their candidate first. The point being that I don't think the GOP that year was the same one that would nominate Reagan four years later and that the race would be more competitive than we'd think on first blush.
The general election would be close, and if Carter is the man as in OTL then I think the GOP can win it. Carter was not an adept candidate and unless you can totally butterfly away things like the Playboy interview or his poor debate performances then it's going to be a toss-up by November. A couple of breaks his way and Reagan can walk away with the Electoral College if not the popular vote altogether.
(1) Ford's two early primary victories, which established him as GOP front-runner, were actually pretty narrow--49.43 to 47.97 in NH and 52.80 to 47.20 in FL.
https://en.wikipedia.org/wiki/Republican_Party_presidential_primaries,_1976 To say that those are poor numbers for an incumbent president is to put it mildly. Remember the fuss when LBJ "only" won NH 50-42 against Eugene McCarthy's
write-in candidacy in 1968! Ford did well in the Illinois primary, but that was never one of Reagan's stronger states, despite being his state of birth. Then came North Carolina...
(2) Remember that not all conservatives supported Reagan against Ford: Barry Goldwater for example eventually came out for Ford:
https://www.nytimes.com/1976/07/01/...by-goldwater-gold-water-gives-support-to.html James Buckley also supported Ford.
https://books.google.com/books?id=ud9bCwAAQBAJ&pg=PA101 I think the reason that some conservatives backed Ford over Reagan was obvious: he was the incumbent president, he had a pretty conservative record in Congress, and as president he was more conservative than Nixon, especially on economic issues. Moreover, his pardon of Nixon, however unpopular with the electorate as a whole, helped him with conservative Republicans. Yet in spite of all that, he lost to Reagan by landslide margins in much of the South and West. IMO no other moderate or moderate-conservative Republican would have gotten anything like even the limited conservative support Ford did.
(3) The non-conservative GOP vote would IMO be much more seriously split than the conservative vote. I don't think Reagan would have any serious rivals for the conservatives. Connally was distrusted by many conservatives as a recent Democratic convert. Crane was a failure in 1980 because he was running as a younger Ronald Reagan--and conservatives were quite satisfied with the old Ronald Ragan. This would also be true in 1976--indeed Reagan's age would be even less of an issue.
IMO Reagan wins the nomination fairly easily. November, I agree, is likely to be close, but on the while I think Carter would be favored (I think Reagan would win a lot of "wasted" votes--e.g., winning California by a landslide which means no more in the Electoral College than Ford's narrow victory there).