There is a port size difference between the magnum port hight and the manifold. The manifold port is longer than the head's port and causes a leak on the bottom of the manifold.
I think he said that he had a 5.2 magnum engine in his car so it should have the larger machined surface with the smog port, but I could be wrong??? I am currently running a 340 manifold on some 302 casting that have the larger machined surface. On the passenger's side I'm running a large port magnum manifold and it seals up just fine. It's been in the car for over a year now.
I also have a pair of older (1969) 318 heads, a pair of 360 smog heads and some magnum manifolds sitting on the shelf so I can easily check if the OP really needs to know if there's a difference???