I currently have blackjack headers,the header flange is considerably smaller than the head port size. All major manufacturers quote the same size, these headers require a smaller than stock gasket. Seems to me this reduction in port size is defeating the purpose. The stock head port gasket is 1 3/8 x 2 1/4. The header gasket is 1 1/4 x 1 7/8. A stock gasket will not even seal the headers. My FE390 is in a 1976 F 100.