Nice build. I'm trying to do something similar... someday... when I have time. I plan to use a flange and the IH gaskets from Riffraff where the price is somewhat justified. The material is tempered stainless steel (spring) that's as thick as about 2 sheets of paper. It's springy so it keeps the seal between the cast iron collector and the steel flange during thermal expansion. Cast and steel expand at different rates and temperatures, so although the bellows take up the expansion movement between the collector and the exhaust manifolds (large movement), the gaskets maintain the seal between the cast and steel (ultra tiny movement, but there). Same principle applies to why there is no gasket between the head and exhaust manifold; both are cast iron and expand and contract at the same rate.
Now as I said earlier, the price is somewhat justified only because of the work the gaskets do. The gaskets are mass produced, and should have a much lower cost. The same gasket for the SD up pipes is only $10 for the pair, so it is hard to explain a 5X difference, but IH sets the price.