Battle of the Runners Up: John McCain vs. Bill Bradley, 2000

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McCain would have been stronger than Bush. Bradley would have been weaker than Gore.

I'll give McCain every state Bush lost by six.

Bradley wouldn't have been a weak candidate.  He'd served three terms in the Senate and he wouldn't lose support to Nader like Gore did. Given how Nader managed to snag around 5-6% in some states, like Maine, Minnesota, and Oregon due to dissatisfaction with Gore, I doubt Bradley would do so bad as to lose every state Bush lost to Gore by six. 

I will agree McCain would've been stronger than Bush and probably would've won the general given he was seen as more moderate back then, but it would've been close. 

Something like this probably. 

McCain: 289
Bradley: 249

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My guess is that the things that would make Bradley more appealing to the 2.74% country that went for McCain, would also have likely sent more centrists Dems into the arms of a Republican war hero considered more moderate than Bush.

A fair bit of that 2.74% is clustered in some of those close states.  For instance, I doubt McCain would win Maine where Gore beat Bush by six AND Nader carried over 5% of the vote on top of that. 

I also doubt Bradley would've lost many centrist Dems to McCain.  While he ran to the left of Gore on some issues, he wasn't far left.  Remember, he co-sponsored the Tax Reform Act of 1986, one of Reagan's tax cuts. 

McCain would've undoubtedly picked up more independents and undecideds and won the election, but I do think Bradley would do respectable as well.