Nixon was actually known as a supporter of civil rights too, he just didn't approve of busing. He wasn't as fierce a conservative as he is often portrayed nowadays.
Wallace was still a staunch Democrat (he stayed a member of the party until his death) and might have pressured his voters in Congress to vote for Humphrey for two reasons: to preserve party unity and to extract concessions to the South on civil rights issues (maybe a few Dixiecrats in the Cabinet).
So I'd say he'd lean towards the Democrats but would offer his support to whichever party made the biggest overtures towards his base.