Drivers header leak

If you want to do it right, once, - you gotta take the header off and "true" the "flange" , by taking a straight edge, and where nec. weld in the low/valleys, - file down the high/peaks.
Look up "draw-filing".
Once the mating surface are "true", you should be able to use reasonably priced Mr. Gaskets that last for years/decades .
Good Luck .