actually.... reagan runs against president agnew, definitely (as he did in 76), and IF he gets the nomination, and that's a big IF, he's just cocky enough to play it straight. he gets a moderate/liberal senator or congressman from a big eastern/northern state as his running mate (ford? hugh scott? charles percy?)
but i don't think mcgovern gets nominated in 1972 if he's not running against nixon. there's no CREEP. there's no canuck letter. muskie doesn't get knocked out for 'having snow on his face'. they've got to move to the left of reagan internationally, but they have no reason to move so far left (remember, nixon was essentially a moderate-centrist on foreign policy). they're not going to go with super-hawk scoop jackson (roughly equivalent to reagan), but considering reagan is law-and-order governor of the state the counter culture started in (and hated because of it by the left already in a way that nixon isn't, although don't get me wrong, they hated him too)... i think they have to go center-left. they go with muskie.
muskie probably does best with a younger liberal from a midwestern or plains state. he picks mcgovern, who put up a hell of a primary fight. other possibilities: mondale, fulbright, or birch bayh.
in the end, it's a tough call, but i think reagan loses in a close race to muskie. the toss-ups are public opinion of nixon and how much sentimentalism sets in after his death. (of course, you could have it be just a bit more realistic and have nixon resign during his first term - which would insure muskie's win).