I've heard this before but I'm not sure about the science behind it. The radiator has little slits that the water has to pass through which reduces the flow rate. If the thermostat is fully open does it restrict the flow that much more than without a thermostat?

 

I bet your intake manifold gaskets are shot. Mixing dexcool with regular antifreeze not only creates all that rusty mud, it rots away the gaskets under the intake manifold.

You say it runs hot. Computer may have shut down fuel injectors and engine is only running because fuel injector spider unit is shot and there's just enough fuel leaking past the closed injectors to run without the computer...but it would be really really lean and run hot. Common for wire harness wires to rub through and short out to keep the computer from opening injectors. It'll run without the computer pulsing the injectors in this scenario.

The wiring harness for these was poorly made. Mechanics wouldn't know the computer isn't even pulsing the injectors to get it to run. They would just see it running and never even think of it. the shot fuel injectors could be leaking enough fuel for it to run and the ICM provided barely enough timing advance without the computer to chug around but running lean/hot.

Laying under the truck and looking up at the radiator, are your rubber radiator mounts all melted and dripped down/burnt? Ask me how I figured this out....

Whatta crappy fuel injection design/wiring harness/computer software GM made for these piles of.......

 

Yes Presti7 they are melted and burnt. I pulled the radiator and fleshed it out and noticed them melted the day before I posted this.