Chunks are bad, wonder what they came from? I assume you got them out of the bottom of the radiator when you drained it? since flow is from top to bottom they would have had to come thru the radiator to reach the bottom of the radiator. could be from a disintegrating coolant pipe coupling, failing hose, excess gasket sealant? at any rate, if that is what came out the bottom, what is stuck on the top of the radiator that was too large to flow thru the passages? I would drain enough coolant to remove the upper radiator hose and borrow/buy a small inspection camera to put into the upper port and inspect the passages. If you find a lot more red plastic, you might be able to backflush it out of the radiator. at any rate you can get a look at he passages and decide if the radiator needs internal work. The radiator of course also needs air flow to function so those passages(and the intercooler passages in front of the radiator) must also be clear. As the engine reaches op-temp the fan should be fully engaged and pulling a lot of air thru the radiator. No flow = no transfer = overheat
Next question I have, is do you have pump output/coolant flow? There is a line that runs to the top rear port of the coolant expansion tank from the pressure side of the thermostat housing. If you remove the pressure cap on the tank and start the truck, you should be able to hear/see fluid being pumped/sprayed into the expansion tank(from back toward pressure cap). Your heater coolant input also comes from the same point on the thermostat housing, so the heater should have hot air as the engine warms. Coolant also runs down the bypass pipe(large stainless pipe in front of air compressor) to the transmission heat exchanger. It should also get warm as engine warms. No flow = no pump = overheat...
If you have valid/unclogged radiator passages and airflow, and you have pump output/flow, I would suspect a stuck thermostat that is keeping the flow from reaching the radiator to be cooled. It is located in the housing on the top front of the engine where the upper radiator pipe connects to the engine... Quick test is to remove it, put it in a pan of water on the stove and warm it above 210F and watch if it opens...
