Help! Done everything and still Overheating

I have a 1989 Chevy scottsdale with 350 5.7 TBI 2500. I have had overheating problems since I bought the truck. I have changed and removed thermostat, water pump, fan, fan clutch, flushed radiator, thermostat housing, hoses. Coolant temp sensor seems fine if I disconnect it, the truck won't turn on, or if its on it will rise rpms. What could be wrong? Any suggestions or advice is greatly appreciated.