[K.Wilson]

Ren Man,
Check to see if you have a "reverse rotation" water pump. A 5.0 (302) pump as used with a serpentine belt will fit on a 289 but would turn the wrong direction with a V belt system. If this is the case you would still have water flow but at a much reduced rate, and would cause overheating.
I went through an overheating problem with my big block only to find the pump impeller was loose and slipping on the shaft. It took two weeks of detective work to locate it.
Keith