Your challenge is to have Spiro Agnew in the Oval Office by 1973, with a POD no earlier than 1970.
Agnew as pres right after a presidential suicide is one of the most dystopian PODs I can think of. He's the worst possible combination, a blusterer who thinks he's always right who can't get along with anyone and will never admit how much he doesn't know what he's doing. To top it off, his own bribery scandal may follow soon, and there will be plenty of demands he resign.
MBR: Now that would be interesting, in the Chinese sense.
AmInd: no reputable Nixon biographer endorses that view, though I suspect it comes from Final Days.
The Democratic field would probably be larger. The problem is that Reagan would be fighting with another Conservative, whereas Ford was more of a moderate with conservative leanings. And with the GOP appearing to be heading for disaster, Reagan might just let Agnew take the fall.Hell, maybe the GOP revolts and nominates Reagan instead. He'd be an infinitely better POTUS, serving at a younger age than OTL. Who would the Dems nominate in 1976? Scoop Jackson's their best bet, all the rest are has-beens (Muskie), second-raters (Carter, Glenn, Harris), or nonentities (Mo Udall).
That seems like the best bet, and with Scoop's New Deal economics Reagan has an open net in 1980. Of course, Iran will turn out somewhat differently with Scoop in office. If there's a hostage crisis, he'll handle it much better than Carter. Still, that won't help the economic situation.