Reagan in '76
Ronald Reagan in 1976 was not as smooth a campaigner as he was in 1980. And he was one hell of a lot luckier in 1980 anyway with all the Iran Hostage business. (Recalling the "backstage" TV coverage of the Republican convention in '76.) Assuming a lot more party discontent with Ford, Reagan could have got the nomination anyway. And have his head handed to him in the General.
Now, a losing "conservative candidate" for the second time would have a tendency to discredit the Right. There was a big surge after Reagan got in the White House, validation and all that. Two big losses, Goldwater and Regan, would send the money men looking elsewhere.
At the same time, Nixon's "Southern Strategy" had already changed the Republican party, so the Big Two would be more equal, and perhaps even more alike. If the Democrats could avoid the trap of the New left they might remain more moderate.
What you could end up with is a Republican Party with lots of conservative, lots more moderates, and a sprinkling of liberals. The Democratic party would have lots of liberals, a good slice of moderates, and a sprinkling of conservatives.
Which dynamic would totally change the elections after 1980.
OTOH, if Reagan DID make it in 1976, he would be the egg-on-face leader who dealt with the Ayatollah's shock troops. (Some may argue the Iranians would not risk a confrontation with Reagan, but at that time he would be an unknown quantity, and presiding over a stagnant economy to boot.) Both of those would virtually guarantee a Democrat in 1980, but just who is problematic.