My response was trying to answer the original question. You can't bolt heavy exhaust to a plastic or foam mockup. They won't hold it. That's why no holes.
So I gave alternate suggestions. Others continued to push the exhaust manifold which can not be done with the original request.

Now the OP has changed altogether and wants to use an iron block. I can not anticipate that an OP will not stick with his initial criteria. I got sucked in this time.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn