Well the Green Party did not form until the early 90's, I believe, so if Nader wants to run before that, his options are somewhat limited, unless we assume he forms a third party himself. He could always run as an independent, something he's been fond of in recent years.
I think the best chance to have Nader elected outside the Democrats is for him to be Mcgovern's running mate in 1972. They still lose in a landslide, but it gives Nader some stature in the party. In the years after that, he gets disgusted with the Democrats move to the right,and runs as an independent for some office in Connecticut. Likely he'd only end up spliting the left wing vote, but maybe he'd become like Sanders and get endorsed by the Democrats even though he's not actually a member of them.
I don't see him being that effective in office. He'd probably end up running for president a few times, losing any unofficial support the Democrats were giving him. Greens are probably a little stronger if they still form, since Nader will have been an office holder, But they still prove unable to break through the two party system.