I built a 350 mag for a friend and had very good success with World 200 cc iron heads. We left the bottom end alone and changed out the top with the new heads, a little more aggressive hydraulic flat tappet cam, a Team G single plane intake, a Holley 800 DP, and headers. The boat gained a lot of low end and mid range and picked up 10 MPH on top end. The boat was a 22' Challenger which is very light weight which is why we used an 800 and a single plane intake. :)

BTW small block chevy guys I have a perfect 2925 Edelbrock single plane intake for sale. See the parts for sale section!!

All good info. You guys are helping me alot. I was hoping to find some flow data on the stock 217 casting heads that are on my motor. Cant find it anywhere. I did find a lot of flow comparisons on the chevyhiperformance.com web site. I have been leaning twords the vortec because of all I have heard on them and the flow data from that website. Now, hears a big wrench thrown into the mix. My local mercruiser dealer tells me he has a 2000 vortec mag engine that has no hours on the bottom end, the heads have been slightly massaged, ie 5 angle valve job, guides machined and good springs, and a custom ground crane hyd roller camand crane gold self aligning rockers. all of it intake to pan, for $2,000. Now what my head is spinning in two different directions..... What should I do? Hippie.