Check the valves on the passenger's side of the engine to see if you've got an exhaust valve on #4 or #6 that's only opening partway. There was a real problem with some engines in the late 70's to mid-80's wearing out exhaust valve lobes on the cam, and it almost always happened near the EGR valve passages. Backfiring was always one of the early symptoms of a flat cam. Leaky header flanges will create a problem also. It would be hard to run fast enough to give me a set of headers, much less make me spend my hard-earned money on junk like that!
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!