Gary Merrill and Chuck Connors in Ride Beyond Vengeance, 1966.
This was the closest picture I could find to any of these actors
actually looking like the parts in question.
THE AVENGERS, 1951:
Iron Man: Tyrone Power. The best-known Zorro had the swagger for it.
Captain America: Henry Fonda. Jimmy Stewart might have been a better emotional choice, but he was too skinny.
Bruce Banner (before the change): Jimmy Stewart. Here, he would be just right.
The Hulk (after the change): Chuck Connors. He was big enough and muscular enough, and it would be too soon for Steve Reeves.
Thor: Kirk Douglas. He would have looked great with the long hair and the winged helmet.
Hawkeye: Humphrey Bogart. He was grumpy enough.
Black Widow: Ingrid Bergman. She could do the accent.
Dr. Strange: Clark Gable. He had the mustache and the style.
Spider-Man: Dean Stockwell. He was 15 then, supposedly the same age that Peter Parker was when he was bitten by the radioactive spider.
Ant-Man: Mickey Rooney. He was so short, he wouldn't need special effects.
Black Panther: Paul Robeson. The only black actor of any note then, and it's a middle finger to the blacklist.
Nick Fury: Woody Strode. If a black actor must be cast, he was probably 2nd to Robeson among them then.
Loki: Ronald Reagan. He was a liar and a trickster with delusions of leadership in real life, so, why not? Besides, I wanna see him get clobbered.