I have replaced the radiator in two ZR-1's in the last year. I went with a DeWitts in one and a Ron Davis in the second one. I believe that the C4 radiator for manual transmission cars is NLA.
Here are some links to previous discussions:
http://www.zr1netregistry.com/forum/...ead.php?t=7737
http://www.zr1netregistry.com/forum/...?t=3713&page=2
Jim