The scary thing about porting is casting shift. It only takes a little bit more and you blow a hole in the side of the runner then your head is toast and the porter says "hey it's not my fault, you should have had them sonic tested before. I told you". And there you go out looking for another to replace it. Of course now you're sitting there with one head all hogged out by someone that is a borderline moron and has no idea what a ported Pontiac head is supposed to look like. So you may as well start looking for two.
Now there are some in the industry that know what they are doing but you pay a premium for that kind of expertise. In the end unless you absolutely want to keep the look of originality and are willing to pay through the nose for it, it really doesn't make sense to port a set of Pontiac cast iron heads.
My 2 cents.