The poly should have water distribution tubes internal of the exhaust manifolds. These are used to ensure water is distributed evenly in the manifold by eliminating any air pockets. They will look like a length of copper pipe. Maybe one is plugged up? When your uncle did the impellers, were thay busted up? I ask since the pieces of impeller can travel plugging some pasages. 

I went back into your previous posts and did find that your 65 did have a problem with rubber impeller pieces back awhile. It may be worth disconnecting every hose and then connecting up the garden hose to each to see if you can flush anything out. I'd also open up the T stat housing and drain plugs and run some water through the engine as well.