Hard to say exactly how much it would cost, couple hundred at least, if you can find someone who can do it in your area. Most are not familar with the 3.4 heads modification. I do machining and I screwed up 3-4 heads before I got the valve areas right, took to much off.
Just gasket matching the exhaust and intake ports will improve performance. Casting flash is what I call it, where the runners are fatter at the edges. Here's a couple pics that'll give you an idea.
Hope that helps.