Look for something that can affect both sides of the engine causing it to run lean with those codes. Most likely it's related to the mods you've done if the codes weren't there previously. A vacuum leak, any source of air entering the engine that's not going through the MAF sensor, a bad MAF sensor itself, a MAF sensor not calibrated for any induction system mods that have been done, low fuel pressure, contaminated gas would all be things to look for to start with.
If you can look at PID's with your scan tool, look at the fuel trims at idle, then look again at 2000-2500RPM. If it's running not near as lean at higher RPM's, you can rule out a vacuum leak as the cause.