The HG's rarely go on these motors, but the LIM gaskets need to be replaced if not running the updated aluminum ones. If you look where the head and lower intake meet the block and see a black edge, then it's time for LIM gaskets. Do a coolant flush and get rid of the Dexcool as this is the source of the brown sludge. Once all of the old Dex is out just fill it up with conventional green coolant and some distilled water and call it a day.
Since you had the thermostat changed, did you bleed the cooling system properly? If not you may have air trapped in the system (which is common on these motors).
