Classic/Antique Car Repair: 1934 Chevrolet Master Sedan: Overheating, cylinder head gasket, exhaust valve seat

Question
I am experiencing overheating and loss of water on long highway driving of 45 minutes or more. I lose above 7 litres of water during this time and of course the engine overheats.It is not the cylinder head gasket as that has just been replaced.
What could be causing this overheating/loss of water?
Would pressurising the cooling system stop this problem?

Answer
This could be caused several conditions. The most obvious could be a partially plugged radiator. The lower hose could be collapsing when driving. There are copper nozzles in the cylinder head that direct cooling water to the exhaust valve seat areas. If these nozzles had deteriorated over the years this could cause overheating after a few miles of driving, then the coolant loss out of the overflow could be expected. Then there is the water pump. It could be sucking air into the system when running. I would pressure test the cooling system to check for leaks as the first step. I am not sure that pressurizing the cooling system without finding the problem is a good idea.