You should use marine exhaust gaskets. You can find them online at any reputable marine store like Hardin or CP Performance. Ron or Dan might even have them available through them.
There are a lot of instances that you can cheat on and use automotive parts in a marine environment. Most gaskets and seals fall into that category, however head and exhaust gaskets I personally feel are the exception. The marine versions have a stainless core vs. the steel core that automotive versions have and are much better in the long run for a marine application.
If you're worried about the fit, you better break out the air compressor and die grinder to port match the exhaust ports in the head to the gasket and the manifold. I also have those same aluminum logs on my SBC and I did exactly that when I rebuilt my engine. I had to whittle away a lot of stock to match everything up but I have 200CC runners in the RHS heads that I put on.
FYI, the unmatched stock heads, gaskets and manifolds worked just fine for 1300 hrs.
