On the headers try some Permitex "ultra copper" gasket sealer. It's good for high temps headers. Pull the headers loose (maybe easier said than done...) and pitch the gaskets in there now. Then lather on the gasket sealer on the header. Bolt it up tight and let it set over night. The next morning (or evening after work) fire it up, idle for a few minutes, shut it off. Go around all of the header bolts again and make sure they're tight. After the first trip or two also check the bolts to be sure they're tight. And also the first oil change or two.
I first did this on the 454 big block in my old chevy trailer towing pickup after it blew every brand of header gasket made. The old chevy's pretty rusty now but the headers still don't leak. I've since done this on motorcycles, old john deer 2 cylinder farm tractors, etc. It works really well. Metal to metal with the "ultra copper" in the middle. Cheap!
later,
scruffy