Hello most definitely we need more info cause it sounds like you got multiple issues with your truck. if your dash lights are still off. Most likely you may have blown a fuse. I would start with disconnect all of the radio wires and check the fuses. Then make sure you wire the speakers and radio correctly. I Agree you need to check the radiator to make sure there is fluid in it and in the overfill tank. I would also agree check the engine oil to make sure the engine did not blow a gasket or if something happen to the engine. If there is fluid and the engine is not cross contaminated with radiator fluid then let us know does the engine run normally till it gets to the hot condition. I do not know if the radiator fan is electrical or not. If it is not electrical check the radiator fan is moving and when the engine is getting hot to confirm the fan it is pulling air through the radiator. When you switched it from v8 to v6 they may not have changed the fan shroud to compensate for the smaller engine.