The actual combustion chamber in the b16 head is larger than the b18c by a couple cc's.
However the b16 head is supposed to flow the same as an ITR head, so it's a trade off of flow characteristics for compression. You can always shave the head or run a thinner head gasket to bring it back up.

Not too sure on a b18c head on a b16 block, haven't really heard of anyone doing that. Not saying it hasn't been done, just that I don't know anyone that's done it.