I would say to run alcohol or don't use the heads. In this kind of racing application the concrete will probably not be a problem.
I personally can't see ever a reason to do this though.
I have been tempted a few times to fill only the intake side of head with a Hard Block and having the head sit on the intake runners to allow the filler to set up in only that one section of the hed. That was becuase of continual pin hole leaks in intake runner with extremely modified heads.
Ed