Phil, I'm not personally versed in how to put headers on a 427 Cobra. In my day they needed to be built on the car.
I remember CSX3121 going that route.
Check with Curt Voght at Cobra Automotive. I think he has done more then one set. I know that he has a header builder, computer and all, calculates everything, that can build them for you. Pricey though.
When you see what he has had there in the shop and probably what he has in the shop currently you won't feel uncomfortable about leaving the car out of your sight. Usually a bunch of GT40's, R's and T/A cars. It's a sight to behold.
It's a lot like the 'Race Shop' at SA back in the day.
I've become a big fan of a completely stock looking street 427. It must be a mature acquired taste? They have great subtle lines. I'm over the S/C thing.
I've also learned to weld on the cast iron exhaust manifolds with stainless rods. I'm wondering if 428cj cast manifolds could fit over the chassis tubes with a little welding, etc?
Lot's of folks have been sneaking them on GT500's that never had them before. Cobra Automotive has been extrude honing manifolds and I think that some of the cast aluminum "cheater" sets might still be around.
They aren't as good as headers but they do help and they sure are much quieter. Kind of fit into the "stock car" thing you have going on maybe?
Shouldn't that car have been built with a '67 2x4 Holley intake?