I'd have to look through my books, but I recall an interview somewhere when Eastwood briefly mentions Lee Van Cleef. A question was asked regarding Van Cleef's inclusion in FOR A FEW DOLLARS MORE, and Eastwood says that he "used to see Van Cleef a lot at Universal Studios" when both were contract players there in the 50s.
Let me preface this by saying that Clint Eastwood is one of my favorite actors. He is a master of minimalism, and much like Lee Van Cleef, he is wonderful with pure cinema--his eyes and his body language are so expressive; so effective at giving the audience emotions and thoughts to read. He is also one of the all-time great screen presences on screen; so much so that the only people I can think of who rival him are Lee Van Cleef, William Smith, Jack Palance, and Gene Hackman. Having said that, here's my take on why Eastwood never again worked with Van Cleef: I don't know how he feels about it now, but I suspect that Eastwood said very little about Van Cleef back in the 60s because he was developing his now-famous persona, and at that time--up until about the time he did UNFORGIVEN--Eastwood seemed to be a bit insecure about himself onscreen; to wit, he was reluctant to share the screen with anyone with equal presence. I'm sure he knew this as Van Cleef stole the show as The Colonel in FAFDM. Likewise, Van Cleef was scene thieving like mad as Angel Eyes in THE GOOD, THE BAD, AND THE UGLY. After the Spaghetti Westerns, when Eastwood came back to the States, he was establishing himself as a major force, and you seldom--if ever--saw him getting upstaged, especially in his 70s and 80s films. It wasn't until UNFORGIVEN where I believe he finally realized himself to be talented and special--even if critics had, up to that point disagreed. After all, Gene Hackman and Richard Harris did steal the scenes they were in. However, Eastwood's world-weary but deadly persona hung bullet for bullet with both Hackman and Harris.
Someone earlier mentioned Van Cleef interviews. There are two good ones available: first, an October 1979 interview can be found with William Horner in his book, BAD AT THE BIJOU; the second can be found--in abridged form, I believe--in Sir Christopher Frayling's ONCE UPON A TIME IN ITALY: THE WESTERNS OF SERGIO LEONE. This interview is an April 1978 interview conducted by Alex Cox at USC. Van Cleef was very insightful, and my favorite quote of his can be found in this interview when he is asked about his approach to acting. The quote goes like this: "A lot of actors think that the more words they have, the more attention they get. That's bullshit. I MAKE people LOOK at me." Classic!