It is best to practice on junk heads. I have a set of old power pack chevy heads off a 283 that I intend to practice on. I also have a set smog heads on a 327 that I can use, too. There are lots of good article on porting. I found one from either Chevy Perf, Hot Rod Mag, or some magazine along those lines. They did a "first timer" porting write-up. Was very good. Especially since the person who wrote the article had not done if before, so you got a very good opinion from a newbie. They consulted with a very seasoned machinist, and had him "look over their shoulder" when they did it. I just, basically, ported the intake manifold I made for my stroker. So I used what I learned from that when doing it. Really helped. Another thing to remember, these heads are aluminum. MUCH softer then steel, which means way easier to **** up, and far less forgiving.
