Lopez had "hardluck" against the top LHeavies of the 70's. (Galindez, Conteh, Saad M.) With a break here or there, Lopez could won a title. Johnson was a great boxing tactician, but Archie Moore had his number. Johnson also didn't seem to have the "fire" in his heart like Lopez did. Could Yaqui have won the Lightheavy Title he was so close in doing in the 70's?
Yaqui had one of the most educated left hands around and the heart of a lion. Lopez could give anyone in the top tier a fight - a very competitive one. But Harold's got more quality on his resume. Yaqui wasn't slick enough a la Willie Patrano to outslick Johnson, and he'd force Harold's hand and make him fight. Lopez being just too easy to hit, that wouldn't work to his advantage. I don't think Johnson stops him, but pulls away to win over 15 by a clear but not overwhelming margin, I'd figure.
Lopez had the heart of a lion...and alot tough luck in a brutal era. IMO Johnson would out jab Lopez easier than Conteh did. Johnson was a great boxer, technician.
Johnson in a clinical performance in which his vastly superior skill and jab would make the difference. This one wouldn't be close at all.