Heater blows warm air

It's been in the teens here temperature wise in Central Missouri.
I have a 1979 El Camino with nearly a half a million miles on it. (Second engine, fifth transmission, etc.)
It has a 305 in it.
The heater blows out warm air but not hot like it use to.
The radiator had a leak and had been fixed several times before so I found a used three tube radiator to replace the two tube radiator. The radiator was flushed and checked for leaks.
I replaced the thermostat with a new one (195 degree I think).
I replaced the heater core today with a new one.
It still blows warm air but not hot like it use to.
The heater hoses are 5/8 and 3/4 inch. The 3/4 inch comes from the top of the waterpump to the large tube on the heater core.
The 1/2 inch hose goes from the heater core to a vacuum operated-valve that closes the flow of hot water to the manifold while the air conditioner is being used.
The large hose seems hot but the smaller hose seems warm like the vacuum-operated valve isn't opening.
I wouldn't think that the three row radiator would be cooling the water so much that it wouldn't give heat at the heater core.
I can't tell if the vacuum-operated valve is opening or not.
I also have a piece of cardboard blocking half of the radiator to keep the radiator from cooling too much during this cold weather.
Any ideas?

Maybe you could remove the valve & get a regular 5/8 hose fitting to put into the manifold. If you get more heat you will know if the valve needs to be replaced or not. If it acts the same at least you'll know it is not the valve.