Well, a Wallace Rockefeller 1976 is very interesting, but there are problems that make it a unlikely scenario.
1) As arleady pointed, Wallace was a very different person to Reagan. I would not call him a New Deal Populist, but I would call him a Populist. He made a carrer based on fighting for the poor southern whites and lower middle class in the 1960's. Then, as the decade went by, he moved to the Racial issue(in my view more out of opportunism than actual zealous bigotry). I do believe he knew how to move with the tides, preaching one stance in a speech, and another in the next. Deep down, however, he was big in social spending. Reagan on the other hand, made a political carrer based on Fiscal and Social conservatism and Neo-Liberal financial policies. He also preached a more open racial stance, attempting to attract black voters. All in all, the profile of the two diverge considerably.
2) The Democratic Party by 1976 was far too gone from it's southern roots for Wallace. Now, regardless of Wallace's apparent racial stance in 76(which, though no longer segregationist was far from progressive) he was still a symbol for racial separation in the country.
The greatest living representation of the classic Dixiecrat. The Democratic Party in 76 had undergone years of change at the hands of figures like Humphrey, the Kennedy's, and many other's who built a base on racial equality and social spending. For Wallace to win the nomination, even in better health(not getting shot does that to you), was extremely unplausible. Let's take a look at the nominee: Jimmy Carter. A Georgian, Carter was a Southern Democrat. His built his reputation on fighting segregation and supporting social spending and racial relations. His views were popular with the democratic machine. The Party also needed a figure of the South in a attempt to savage the region after the succesfull Southern Strategy. This played a large part in his nomination. Had Carter not run(unlikely, he was a political climber) for some reason, it's unlikely the Party would've picked Wallace, even with the southern appeal, as he would completely alianate much of the northern Democratic base. A base needed. Imagine he was picked, the South might be carried, but New York, Pennsylvania, Ohio? They almost certainly would go to the other candidate(in this case Rockefeller), and that's enough for a Republican win. Therefore, the Democratic stablishment would avoid such a poisonous figure outside the south like a plague.
3) The Republican machine in 76 was far too conservative for Rockefeller. The VP was a veteran politician, deeply experienced, and a known moderate Republican and a progressive on many issues. This made him a serious pariah to the Conservative wave that begun to swept the nation after the first Oil crash. Ford liked Rockefeller because he was a excellent administrator, but he was not the best political player, lacking interest in party disputes. He could also get easly frustrated(remember he actually flipped the crowd in one instance where he was campaigning for Ford). By 1976 Rockefeller was arleady thinking of retiring, and than came the elections. Ford, who was far from a moderate, was seen by conservatives like Reagan and Goldwater as such. He barely beat Reagan in the Convention, and that only came from the fact that he had agreed that Rockefeller would not be his VP choice for 76. That was how America was graced with the figure of Bob Dole, a compromise choice for the slot that pleased Reagan and the Conservative Wing. Now, if Ford was assassinated, and Rockefeller became President in 75, he might choose to run. He would almost certainly, however,be defeated by Reagan in the convention. Now, there is the possibility sympathy for the VP might earn him a nomination, that is true. I find his nomination very unlikely, but not impossible.
All in all: Wallace becoming the Democratic Choice: Almost impossible. Rockefeller becoming the Republican Choice: Highly unlikely.
Move the scenario to 1960 or 1964, however, and this becomes much more plausible. This is my take at leat.