We have received several sets of aluminum heads to C.N.C. port from customers in which they have cut way too much material out and the porting breaks through into the over-clearanced pushrod tunnel. This can be repaired but it is neither easy nor inexpensive.
Contrary to popular opinion, pushrods do not need clearance. In fact, it is a positive situation when the pushrods touch the heads. It reduces the potential for harmonics and flexing. The question is, ”how much and how hard can the pushrod touch and still be ok?". We test this in our shop using masking tape. We run a single layer of standard masking tape down the side of the pushrod. We then install it and rotate the cam several revolutions. If the tape does not tear, the clearance is just right. If the tape tears, you need to remove just a little more material.