exhaust backfire

I has been my experience that valves just dont stick at idle and when they do they break a rocker arm and/or bend push rod which is very easy to find.
When you replace the header gaskets use Percy's dead soft aluminum gaskets they are great and I have never had one leak. If possible chase the header bolt threads and clean the threads good and use new header bolts.
IMO I think you may have a combination of issues.
Are you runnig an MSD ignition?