I don't see either.
Nixon wanted the presidency. He was getting too old for the time (ironic given Reagan became president at the age of 120). It was either 1968 or 1972 if a Republican lost in 1968, and if the Republican won in 1968 and it wasn't Nixon, Nixon was out. In politics, everyone has a window. Once it's gone it's gone. And 1972 would be difficult, even if Nixon allowed for that leap of faith, because Nixon will have been a loser for everything since the election of 1960. Nixon also came out of the wilderness once in 1968. He can't come out of the wilderness again. Once he's gone, he is gone for good. So in short, he won't take the vice presidency because he wants the presidency, and because the vice presidency is not a springboard, and frankly it'd be embarrassing I think.
Frankly, and this is a scenario I've thought of using myself for a *1968, I think a better running mate would be Ronald Reagan. Romney is like a Lite Rockefeller; he's not as [word for love him or hate him] as Rockefeller, but he does need to shore himself up with the Conservatives. Nixon didn't really have that problem as much because he was someone the Rockefeller and Goldwater wing could agree on and accept. Romney won't have that problem like Rockefeller would, but he would be smart to shore up support. There is the problem that both men have the same problems: dunces who look pretty. I like Reagan and I like George Romney, but they were a bit ditzy, and Reagan was a very shallow person. Reagan was just a handsome, popular guy and he had no depth and was shallow as handsome, popular guys tend to; not anything malicious, but just the way it was. To hear some thing said about Reagan, he comes off as borderline autistic. But I'm getting off track. To summarize, I don't think those problems would have kept them from winning, and they could have won on looking presidential and coming across as true blue and all American patriotic. It would have been a major criticism of them by those that would criticize them though: a shallow ticket for people who wanted to wave flags.
If not Ronald Reagan, than another Conservative seems necessary to be placed as running mate.
Robert Kennedy will not run with McGovern. Kennedy and McGovern don't really get along, as Kennedy is running as what McGovern got into the race to be and some words have been said. McGovern also is not a good candidate. He was not going to win the nomination, and I doubt he'd be placed in the VP slot. Who RFK could run with is up for discussion.