Looking at your photos you already will have a fair amount of mismatch between exhaust port size and header port size. Since the head parts are smaller than the headers that won't hurt you as much as the other way around. I assume the gaskets you show are the ones that came with the headers. I've found the quality of the gaskets that come with no name headers can be iffy.
Might be worth looking at a local auto parts store to see what else is available in name brand gaskets. There are some pretty nice header gaskets out there that'll compensate for a lot of problems. The previous suggestion to use stock gaskets may be a workable solution too.
One final thought if you're so inclined: build up the surface of the header flange with weld and then level it out flat to provide more surface sealing area. I did this on a set of home made headers that wouldn't seal up till I created more 'meat' in that area. Toughest part was taking the time to get the mating surface flat again.
You did buy those right. I'd give them a try. If they don't work out they're good swap meet material.
Yeah, the gaskets shown are what came with the headers. I didn't even plan on using them out of concerns about the quality, but it looks like factory style gaskets won't even fit because of the over-sized ports. Mancini's is right down the street from me, so I'll take one of these and check it against different brand header gaskets.
As you recommended, I'm probably going to have to build up the sealing surface. My engine builder should be able to mill it flat. I just need to find someone with a tig welder.
