I used Martin Senior (NAPA) BBQ Black on my race cars for years with great results. Very high sustained exhaust temperatures. Even though it is a pain, the key is recoating every few years before they rust. Goodluck.
THANKS TO: A&M Machine and Fabrication, CCTek (http://www.candctek.com), Hermance Design(www.hermancedesign.com), Paradise Road Rod & Custom, Harry Opfer Welding, Wegner Automotive Research, Clayton Machine Works