The exhaust manifolds aren't an issue with breathing, it is all in the heads themselves, requiring a good porting of them along with a little work inside the starting points of the intake runners themselves to get more power. Opening up the exhaust isn't necessary given you won't hit the rpm ranges a set of headers work best, and the electronics/smog package requres the engines to maintain a certain amount of back pressure in order to run clean, which can be an issue in many places these days. With these things in mind, too, a mandrel exhaust pipe instead of a crimp bent pipe will help on that side, and it is a lot cheaper and easier to reinstall than headers. Back to the heads, where the initial breathing and power making starts, although you have 1.92 intake valves, which are large enough for a street 318/5.2 4X4, the ports themselves have enough restrictions in them that not nearly enough of the port volume is utilized, and if the 5.2 heads are the same as the 3.9, there is a shark fin that reduces flow even more. I ported my heads, removed the shark fin in the intake runner, and removed the sharp edges in my combustion chamber and my 3.9 is pretty stout down low and on top where it used to lag a bit, everything else is totally California stock and smogs in low single digits to boot.
