Young wasn't as good vs fast slick boxers like he was vs the boxer-punchers like Lyle, Shavers, and Foreman (and the version of Ali he faced was neither slick nor fast)
Johnson by split for mine. He isn't going to like Young one bit, & will struggle to string consecutive rounds together. Still, his class (he was a cut or two above Young, but a full division smaller) would win out.
I would not be shocked, however, if Young beat him. Both are under-rated, but Johnson especially.