Well when he retired from the Senate, he was preparing to run for POTUS in 1972. He did not have a care about direct continuation of sticking in public office that isn't the highest office in the country.
I'd say it'd be more likely he run in 1974 to re-establish himself after having done so horribly in 1972. If he does that, he'd have the issue of having to face off against the Democratic incumbent Governor Wendell Anderson. In this situation, and knowing McCarthy's own political beliefs were in flux. So, what if he ran as a Republican?
Maybe it'd be a little early for his beliefs, but with the right PoD he could be knocked straight into the conservative Republican column. Not sure if he could win the presidency in either party if he even won his Republican run for governor. He'd be a political outsider across the entire political spectrum. He could be the result of a Democratic version of Watergate in a Humphrey wins '68 scenario. It'd be tough of him to win in the Party of Nixon, with his anti-war, non-interventionist proto-Libertarian leanings.