1) Someone who can act 2) Someone who could enunciate Shakespeare 3) Someone photogenic, who would look good when up against Antony. 4) Someone who could play a tortured hero and a noble loser. I'd go back in time to recommend Richard Burton.
By 1970 would Burton have had the discipline and control over his talent (and his drinking?) to do the part well? He was good in "Anne of the Thousand Days" around the same time and in "Equus" a few years later. If Burton wasn't too far gone with the high living and hard drinking then he would have been perfect. My choice would have been the late Paul Scofield.
IMHO, Burton for Brutus. Peter O"Toole for Cassius. The young but up and coming Anthony Hopkins as Antony or Albert Finney. And Laurence Olivier for Caesar.
Burton of 1970 seems to be the best choice as Brutus.
Failing that, Burt Lancaster, maybe? He sometimes displayed an admirable smattering of acting chops, e.g., the German authority-type who was tried as a Nazi war criminal in "Judgment at Nuremberg."
Secret Message, HERE!-->CONGRATULATIONS!!! You've discovered the Secret Message!
I don't know. Burton seems like more of an Antony to me, and I'm guessing I would have preferred him to Heston in that role. I don't know who I'd cast as Brutus, although Scofield sounds like a pretty good idea.
Oh yes Burton would have been perfect, he was good at tortured and he had a beautiful voice. The guy they had as Brutus just seemed bored the whole time, he wasn't right for the part and he sure didn't seem noble or tortured.