Best way to cure a header flange leak on any thickness flange is to cut the flange with a hacksaw to within 1/4" to 3/16". Leave the flange "tied" together so the flange doesn't spring apart and mis-align, causing more problems with hard-to-start bolts. Remember to use a good quality gasket, not "Cheepo-Gasket Works". Whenever possible, I'll use a Mr. Gasket "Black" gskt.
After you get the header tight, look at the flange. It won't be straight. The flange will bent and twist to fit the head. This really works on small and big block Chevrolet, and anything that has flange material between cylinders.
I've used this method for years with great results.