Unless the header flanges are at least 3/8" thick (1/2" is better) leaky header gaskets are simply a fact of life. At full throttle the temperature of the exhaust gas leaving the cylinder head can be as much as 2,000 degrees F. There's a reason factory exhaust manifolds are made of thick cast iron. 30-something years of building race engines has taught me that there is no magic sealer that will prevent header leaks- - - -just keep a good stock of gaskets on hand and change them frequently. Some people use two gaskets stacked on top of each other to help seal slightly warped flanges.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!