So all you did was put headers and now it runs rich? and it was fine before? Does this truck have the original catalytic converter? Do you have a 195 degree thermostat? Is your air cleaner dirty? Are you running the correct AC Delco spark plugs? I believe they should be R45TS but the auto parts store can look them up. If you don't already have one, you might try going to a K&N high flow filter element.

 

An exhaust leak before the catalytic converter can cause a fuel injected engine to run rich. Oxygen from outside air gets pulled in through the leak and results in the O2 sensor reporting a lean condition.