I’ll add this in regards to coolant leaks on fresh rebuilds.  The threaded holes on the ’56 and earlier heads where the intake manifold studs go into the heads to hold the intake manifold turtles (hold downs) in place are open to the water jackets within the heads.  If sealer is not used on the studs then coolant seepage past the threads can happen.  Because this coolant basically weeps out and travels down the length of the head alongside the valley cover, it very often looks like a head gasket leak and is difficult to track back to its source without a pressure tester.  The ’57 and up heads went with a blind hole at these locations so that problem isn’t prevalent on the later model heads.