Here is a before and after, using the propper manifold with the filled heads. The alloy inserts in the head would be the way to go if you have the option. If you were quite handy, most of the work could be done at home.
I have a set of the Parker port stuffers in my heads and it made a huge difference in the low-mid range. It really picks up sooner and doesn't have that "2 stroke" feel to it. I never dynoed it before but I did after and it made 329 rwhp. There is more in it, I've changed somethings and will be dynoing it this spring again. My build up is here if you're interested. http://www.supermotors.org/vehicles/...il.php?id=6723
1982 F100 2wd-408c CHI 3V 225cc Heads, 3v Intake, Pro Systems 1000+HP carb, email@example.com w/ 0.694/0.697" lift solid roller, JPT Rollerized c6 and 5000 rpm converter, 4.86 gears and S&W Ladder Bar kit with QA1's all the way around.
firstname.lastname@example.org mph 1/8 mile with lots of bounce - with old engine and suspension(should be greatly improved) 1982 F100 408c 3v