Physically, Mitchum might not have been a match for Halsey, but character wise, Mitchum was an excellent choice. Spencer Tracy was already dead. James Whitmore, who played Halsey in Tora, Tora, Tora, would have been a good choice as well.
James Stewart was not in Midway, at least not that I've seen, and I saw it in the theater, and dozens of times since. It was the first movie filmed and shown in "sensurround", theaters had to buy and install special equipment to show the movie when it was first released. I think the other movie filmed and shown in "sensurround" was A Bridge Too Far.