I'm wondering on the quality of the water pumps that are being produced. I have turned back a few so far, due to the impeller not being pressed on all the way. There is a large gap on the other ones I have seen too. I'm looking for info on the water pump specs to see if its correct.
I would think a damaged impeller would throw the unit out of balance along with gouging or breaking the timing cover and or pump housing. Do you have good coolant flow when the engine is at operating temperature? Have you pressure tested the system? Russ may be on to the cause, with the clearance of the impeller to pump body not being correct.
You might check to be sure the carburetor is properly torqued to the manifold, there are no vacuum leaks, the static timing is correct and advance weights at the top of the distributor are free. Be sure the vacuum advance unit is working with the point plate advancing freely. It wouldn't hurt to put a few drops of motor oil on the point plate wick and some oil down the shaft up by the advance weights. Lightly lube the advance weights so they pivot freely. Retarded timing from a damaged vacuum advance or stuck centrifugal advance could cause overheating. Good luck, let us know what you find.
I would check thermostat, make sure it is right side up, and is opening up and you are getting good radiator flow per shop manual test procedures. Also, make sure heat riser on bottom of left/driver side exhaust manifold is not frozen shut and is opening up. You can visually see one working in this video: http://6364cadillac.ning.com/video/from-jason-s-garage-heat-riser-o...
As far as overheating, how are you determining this? Temp gauge, IR heat gun? The temp sending units go bad and the new replacements are often miscalibrated. I keep an infrared heat gauge handy. My 64 CDV runs about 195 degrees at top of intake when fully heated up.
Before dismantling your engine and replacing a lot of parts check the basics first.