brians,
I believe the gaskets for those 2 items should be the same as the automotive counterpart, but I will verify that today now that it has come up. There are always options when it comes to sealing, such as cutting your own from gasket material or just using high temp silicone sealant. I found an easy way to pressure test the manifolds, just take the front cover from each of the manifolds and place both on one of the manifolds fore and aft, sealing with silicone, making sure to wait for the seal to cure, about 12 hours. Take a standard cooling hose and connect it to both of the cover plates with hose clamps, this will seal off the water chambers within the manifold. Then screw in a 1/4" npt fitting into the drain hole of the manifold and apply air, about 15psi. I use an outdrive pressure tester that I rigged up several years ago with a low pressure gauge and hand air pump. If it holds the air then the manifold should be ok. The only thing this does not check is the aft risers, but if there is problem down the road this should make it easier to identify and correct. Good luck with you're rebuild.
Jeff