from what i see here in this link, headers are allowed as long as you have the emissions hooked up they should be fine. and did see slp headers for GM trucks in the list as approved. they seem to allow just about anything you want as long as it passes a smog test and the obd2 scan.
Database: Aftermarket Parts Database of Executive Orders
sencond link did not work, got to the top link here, then go to the second drop down search box, look for "Exhaust System / Exhaust Modification" then search.