Buddy;
It is not unheard of that a pump can have its impeller loose on the motor shaft or even be eaten up with corrosion. In that case, the water inside the pump housing is just being slung around inside the pump with little external flow into the engine. Can you drill & tap an 1/8" NPT fitting into the pump outlet for a pressure gauge and check that the pump is actually pressurizing the block with water?
Regards, Neil Tucson, AZ