I have a 96 GMC, fresh crate engine installed in September, the engine is running too cold as per my mechanic even for winter weather in NY. It was running at 199, was told normal op temp should be 210.

put in a new thermostat, radiator cap and a coolant temp sensor and now its running 190-195. Colder than before.

I see vans and trucks all the time with what appears to be factory or aftermarket covers over the grille but I have no luck finding one for a 69 Yukon,

Years ago I had an E350 that I would slip a piece of cardboard between the grille and radiator but the Yukon isn't open behind the grille so the cardboard trick wont work.