Cardboard over the radiator. Engine barely to temp

[INSIDIOUS '86]

So 80% of my radiator is covered by apiece of cardboard. It's a 400 block and runs a high compression ratio and healthy cam the thermostat is in working order and is a balanced 160* I have a flex fan that's 50% under driven
I've never had overheating issues or issues warming up before winter. Before this winter I had the cooling system flushed and it came out pretty clean. The heater core works fine and no clogs and routed right but the engine hardly got past 120*130* water temp. And yes the gauge works and is new sunpro.

Anybody here have this issue or prob? The cardboard fixed it and it stays at 160 and warms up much quicker.
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