If using the old style (large) thermostat housing NAPA sells cast iron housings (maybe from Dorman?). I think they are less prone to warping. You might google Real Gaskets, an outfit in Tennessee that makes silicone gaskets. I haven't had much luck with some of ther stuff (valve cover gaskets), but their thermostat gasket is great - if you try them FOLLOW THE INSTRUCTIONS.
I've had 2 of the "Real Gaskets" blow out and make a hell of a mess, 2 gaskets, 2 separate occasions. It seems they do not like high flow stats.