I think when you take into account the fighters he fought during his career a number of them were in their prime at the time he fought them with names like Eddie Machen, Zora Folley, Cleveland Williams prior to losing his kidney in 61, and he held a victory over a young Ernie Terrell. Bethea clearly had a top notch chin. He also held wins over Jimmy Slade, Bert Whitehurst, Paul Andrews, and a post prime Ezzard Charles.
I dont think Bethea's chin was the reason he only got stopped once. I think it was more due to his style. He kept a high guard and would work his way inside and then maul on the inside. Guys like that are hard to land clean on. Just going the distance isnt indication of having a good chin.
Well, his chin may have been durable, but his teeth's durability wouldn't make it into the top 100 best molars list...
Bethea was a very good journeyman, who, similar perhaps to Jesse Ferguson, was a real Boogey-Man. Aside from the men you mentioned he beat in your post, he also burst the bubbles of Joe Bygraves - who was riding a crest, having just won the Commonwealth heavyweight title and arrived in NY to further his chances of a world title shot - and Franco DePiccoli, the undefeated (25-0) Olympic Gold Medalist from 1960. He stopped them both in 5 and 4 rounds respectively. Definitely a Boogey-Man.
Marion Wilson is the guy who always springs to mind when these discussions come up. All those other guys on the list had excellent chins, but they were still stopped or dropped for one reason or another. Marion's chin, body (which never let him down, either), and durability was something else.