There's nothing unique about iron heads, they port the same as aluminum (they're just harder, obviously). As far as what gains you would get? No idea. You'd have to do the entire package (valves in included, as Tony mentioned) to likely see any gains. You've got a 60+ year old engine design, without much in the way of aftermarket. I'm sure there are improvements to be made, but it's going to take a lot of time and engineering to get much.
1983 J-10 - 4.6L(MPFI)/CS130D/Hydroboost/NV3550/D300/44/44/3.54/Disc-Disc/32s/42 gallon 'burb tank