How much extra porting? If I did anything, I would put in a bigger cam. At over 400 inches it will like it. Both duration and lift. This is a drag car only I take it, so why not put a real cam in that thing. It will make a lot more power if the heads ported, and the intake needs work too.
650hp should be doable with that deal with a bigger cam, and some work to the intake, and a 4150. I did a 406 deal not long ago, that I freshened and tweaked for a guy, it had some older Brodix track 1's, that had what I call scuff and buff porting, where, they port match, do the bowls, and clean up the runners a bit, then sand everything, and call in fully ported. I ported the heads, re-did valve job and freshened it. With a 274-282 .680 lift solid roller, 950 cfm worked 850 carb, on a HVH manifold, it made 655 hp and 570 ft/lbs.
Frank