If I go from a 5.7L V8 to a 5.0L V8, what all do I have to change out ?

You would need to use the engine controller for a 5.0L of the same year as the 5.0L engine and the truck. If your check engine light is on, the controller is trying to tell you that there's a malfunction in the engine and emissions control system, and that could cause your truck to use much more fuel. That the light is on indicates that there are data and trouble codes stored in the controller which could be accessed by a mechanic with a scanner. I recommend that you do that instead of replacing the engine with a smaller one. If you still want better economy, sell the truck once repaired and get a smaller one. Here's a directory link for you: http://repairpal.com/chevrolet-repair-in-redfield-arkansas