You can swap a 16V head onto an 8V block. You will have to use the 16V intake too though.
People also suggest using the same pistons as the head you are using, since there are different valve reliefs cut into the pistons (2 for the 8V, 4 for the 16V). I don't think that you will run into issues (valves hitting the pistons) if you don't swap pistons......but I don't know for sure.
