I'm in the process of porting a set of 241 heads. If you can get your hands on casting 243, use those as those are LS6 heads and flow naturally better than stock (+about 12RWHP stock according to a book i have.)

Porting LS1 style heads is tricky to say the least.. The only things that should be touched is smoothing hte valve bowl slightly, Widning the intake runner where the injector sprays, taking a small bit from the left over casting material, and the most work should be done around the valve stem.

As a guide, ask someone who knows their LS1's well. They'll tell you to take as little material away because the stock design flows extremely well. If you take too much away, you'll increase CFM/ and intake/exhaust CC's BUT LOWER AIR VELOCITY and that will negatively affect your horsepower.

SO if you're doing a home port job like me, DO NOT touch that bit of material behind the spark plug in the combustion chamber, in fact dont grind the combustion chamber only smooth/polish it. Dont get grinder happy, a little caution and holding back could give you the 10-15 more HP than the other dude who screws up his heads (maybe me we'll see).

Some good used heads can be got for 200 ish while LS6's go for around 500 or 600. Stay away from ebay. Alot of times those guys pressure test a set of heads they were going to use in a core exchange and the cores get rejected because of porosity problems or just bad heads. get them tested, then flow bench them, and after you do one cylinder, flow bench again to see if you're helping or hurting.

porting the stock TB is easy. It took me about 3 hours. careful with the grinder again, dont port all the way through the material!

hope that helped...