Aluminum Intakes dont seal on X heads

I am forced to run a cast iron manifold that came on my 340. I am sure the block is decked. Pistons stick out about .020. Every manifold I have tried has not been able to seal at the bottom of the port. The top seems to be tight and the bottom has a gap. I not sure how I should measure to have the manifold cut to fit. the heads dont appear to be the problem. I have even checked the angle on my cast iron to the aluminum and they appear to be the same.