What was the last election where both candidates were significantly different?

[Jerseyrules]

I would say 1892, IMHO.  Or maybe 1960, because Nixon and Kennedy weren't moderates; JFK was a (fiscal and military) hawk and tax-slasher with a monetarist streak, Nixon a tax-and-spend progressive, the last progressive Republican and first since Hoover, IMHO.

[Jerseyrules]

Ideologically/position wise? Probably 1972.

Eh, considering Nixon wrapped up Vietnam fairly quick, his administration probably wasn't too different from the hypothetical McGovern one.

have a look at the rest of the McGovern platform sometime.

I do believe that Nixon was one of the most progressive presidents ever, establishing the EPA, campaigning for national health insurance with a PUBLIC OPTION (which was turned down by the democratic congress thanks to Teddy Kennedy), and restricting oil drilling in Alaska, as well as hiking the minimum wage and enforcing wage and price controls.  McGovern would've done the same thing if he could, but wouldn't have had the political capital to because of his unpopularity within his own party.