Here is an interesting thought.
I see you have the aluminum passenger side exit water pump, and yet your top hose coming from the thermostat is also on the passenger side of your Cross Flow radiator.
Like it is circulating the coolant through the passenger side radiator tank and back into the engine, without it circulating through the main part of the radiator.
Here is a picture of a '76 D100 truck radiator and shroud with the upper hose on the driver's side allowing the coolant to circulate through the full radiator.
View attachment 1716088985
☆☆☆☆☆
