Henry Fonda perfect as Tom Joad, but who else might have been cast?

Great movie and stellar performance by Hank Fonda. I always thought he deserved the Academy Award for this, the role of his entire distinguished career in cinema (his best friend, Jimmy Stewart, took home the Oscar instead.)

But what if Fonda had for any reason been unable to be in THE GRAPES OF WRATH? Who in all Hollywood would have been a worthy replacement? Probably nobody. But I'm going to drop a name, anyway, as my pick for a "second best" Tom Joad:

Gary Cooper.

Whom would your pick have been in Fonda's absence?