First things first, Burp the system just like DusterDude said, don't worry about a finding a shroud right now just burp the system and make sure you have a spring or coiled up coat hanger in the bottom radiator hose. While driving at speed the bottom hose will suck it self shut and overheat. If your still having problems then flush the whole system and if you want better cooling without finding a uncracked a shroud just replace the stock 4 blade fan with a 7 blade fan from another slant that came with AC, you can usually find them for no more tan $40 (the V8 ones are too thick). I did the 7 blade swap on my 73 and on my wifes 66 daily drivers and never had a problem even in summer when it was 102* in stop and go traffic.
Also take to Slantsixdan, there may be some other cooling tricks/issues for the aluminum blocks that he knows about.
