An idea I've had for a while. Still a work in progress:
Avengers Dream Cast (1965):
Captain America - Steve McQueen
Most of my choices (due to a lack of knowledge of 1960's actors) come from the Great Escape, Dirty Dozen, and Kelly's Heroes. Every time my mom saw me watching the Great Escape she would always say that Steve McQueen was the all american boy at the time. When she said that I immediately thought of Captain America and the inspiration for this cast.
Iron Man - James Garner
Also from the Great Escape Gardner gives off a charm that just scream Tony Stark.
Wasp - Audrey Hepburn
Mainly because of her looks, I can't recall seeing in anything but I know she was a top actor back then
Black Widow - Natalie Wood
Basically same reason as Hepburn
Nick Fury - Clint Eastwood
Come on this is a no brainier.
I had more ideas but can't remember them right now. I was thinking Charles Bronson as Wolverine and maybe Donald Sutherland as Spider-man. Feel free to add you're own