Here's an Interesting one: James K. Polk. Retired after 1 term due to a promise he made (out of a fear he might die), and indeed he did die 103 days after he left the White House. He died in 1849, and would have lived to 1875 under this system. He was from Tennessee and I like to think he would side with the Union, given the very split opinion in the state. He might have supported the National Union Party, maybe this Tennessean gets the VP slot in '64 instead of a certain drunk tailor? Nah, but he might be a useful Democratic voice in fighting against the Copperheads, and might butterfly away Johnson's VP nomination.