I made those out of 321 stainless.
If you end up using manifolds, open up the hole diameter on the end ports for the bolts(front and rear). Also use much longer bolts or studs. Add at least an inch longer fasteners, and make spacers to make up the differance in length. This allows for some stretch in the bolt,and prevents breakage. Manifolds or logs have to float on the head surface. The shape of them wont allow for thermal expansion without movement. This is another reason not to use gaskets... You'll just scrub the gaskets away, and warp the manifold/log.I also strongly suggest using grade 5 bolts or lower, and never use stainless bolts with stainless nuts. Tighten the center bolts as you normally would, and just snug up the bolts on the outer ports.