In an interview, Clint Eastwood has said that he was offered the roles of Superman and James Bond. Apparently, the Superman offer was when the idea was first surfacing, so it would have been well before Christopher Reeves involvement. The Bond offer came after Sean Connery left the roles, though it isn’t clear if this was after You Only Live Twice or after Diamonds are Forever.
I honestly can’t see Eastwood in either of those roles, and I think he made a good choice passing on them. Eastwood is certainly versatile, but I believe he does have a type—if only because of his looks and his build—and I really can’t see him in the Superman costume. As for James Bond, it would have been a hoot to have Eastwood as Felix Leiter, but not Bond.
Even if Eastwood could do a perfect British accent, which is by no means assured.
You can read the interview here.