exhaust backfire

Thanks everyone- Only a few hours after my post and now I already have some things to look into!

Halifax- I like your idea of checking ignition with an oscilloscope. I will look into that.

Inertia- It never pops on decel or going down hills. It always backfires at idle. Also, when I first start to accelerate, it often backfires. Exactly the moment the throttle blades are eased open.
I suppose I could coat the copper header gasket with hi temp RTV, but I hate to make a mess of it if that isn't the problem. There is not much room to work in there. I have had header leaks on many cars (actually every car with headers I ever owned LOL), and have never had it cause a backfire. Even this car specifically has had worse header leaks in the past over the years, and it never backfired. So that makes me skeptical that it's the problem, though of course it may be. I will keep that possibility in mind.

Dana67dart- I agree with you that it could be a valve that only sticks when it gets hot. If that is the case, perhaps the leakdown test was normal only because the engine was cold?? Maybe the valve simply wasn't sticking because the engine was cold?
So what next? I think it would be near impossible to do that leakdown test on a hot engine (getting around those 2" primaries when they're hot, burning my hands, melting the leakdown tester hose, etc...)
Even pulling a head- maybe everything would seem fine because the head was cold...?
I also wondered if the electronic ignition box was the culprit, but it seems less likely since it only backfires out the passenger side. Anyways-you asked if it idles rough at the same time it backfires. Yes. It backfires when it's warmed up and the idle does seem to get worse as the engine gets hotter.

Thanks again for the advice guys. Much appreciated. Any other advice is welcome! I will keep you posted as this progresses.
-Jay