Here's a more organized scenario:
1. Gregory Peck runs for Governor in 1970 and wins;
2. His Governorship is more or less successful, or at least not troubled;
3. 1972 Primaries: Arthur Bremer shoots George McGovern rather than George Wallace; Wallace goes on to win two or three more states, perhaps taking enough votes from Henry Jackson to de-rail his nomination, while the left now lack a strong candidate, so a draft Gregory Peck campaign is started late in the primaries, but perhaps just in time to snatch California and some other states; Trapped between Wallace, Jackson and Humphrey and other favorite sons and unlikely candidates, Gregory Peck is nominated on the first or the second ballot, recruited by an ailing McGovern or Ted Kennedy.
Peck takes Sanford or Reubin Askew or Birch Bayh as a running mate (depending on how much leverage the conservatives and the party bosses have in the process, and who is willing to play second fiddle to a Liberal Hollywood star), and there might be a George Wallace retaliatory Third Party Run, which siphons votes away from Nixon.
4. Peck v Nixon v Wallace gives Peck a victory;
There are some holes in this scenario though, might need some patching up...