Make sure that your headers have a good seal to the cylinder head by using a high quality exhaust header gasket from Cometic.
Cometic custom gaskets can accommodate virtually any engine configuration from naturally aspirated, to the most exotic and nitrous injected engines.
GM Dart/Brodix 4.500 Bc Block - .040" MLS H/G 4.270" Bore
Dodge 6.1L Hemi 4.100 Bore - .051" MLS Head Gasket
Ford 4.6L V-8 Right Side 92mm - .030" MLS Head Gasket