Judging by an image search, at the port-manifold face there is a lot of metal. Further in the port, say 1/2" or more towards the valve, the water jacket comes into play.
The two links below show how far you can and can't go. Although both seem to be seriously over ported to me. Gasket matching isn't a good basis for a template.
To me, your head looks ok. I think if it were a problem, then it would be a problem common to all marine vortec head equipped small block chevys. (I am assuming you have a vortec head. The internet seems to think so, too.)
http://www.pacificp.com/forum/viewto...c2a63762107ee9
http://www.justinb.net/bulletproof/page4.html