Who rebuilt the engine? If it wasn't you I'd be on the phone wanting my money back. Wonder if a head gasket could be blowing into the water jacket. Was the blocked boiled out, perhaps a blockage somewhere.
That block could be odd or even fire. Heads were of even fire first year.
Is it a even fire crank And distributor ? Vacuum advance distributor ? Is this a aftermarket 4 barrel carb?
What was the rod main bearing and most important thrust bearing clearance? Is it a automatic or stick? If it’s a stick do you have clutch free play? V belt setup? Lots of questions but a mismatch set of parts can make you life missable. Just count your lucky stars it not a Ford
Remove the thermostat and run it with the upper hose removed from the radiator to prove there is water flowing through the block to the heads and then to the manifold. If there is no flow you may have mismatched heads and block. If the lower hose blew off, that may be the case. If you place the head gaskets against the block and then the heads you should be able to see if there is a mismatch.