Puk'in Fluid

Once the engine is hot,, putting on a pressure tester will tell if cyl press is going into coolant...

and this is wayyy out there,, but you mentioned the waterpump vane difference..

Well a marine pump can be reverse rotation,,, and will cause a problem at speeds over idle,, ask me how I know... it's not likely,, but.... ???

(Folks decided that over $125 for a "Mercruiser" small block water pump, can be replaced with $30 automotive pump,,, it'll idle outta the harbour,, but anything over 2000 rpm,, overheat... lol.. wrong rotation... seen it a few times.. )