I'm in favor of X-pipes, but it boils down to the combo and carb tuning to get the most out of it. With my TTI 1 5/8"-1 3/4" step headers I added a 3" X-pipe with straight through Dynomax mufflers on my Dart Sport and it picked up a bit, but I don't recall the exact numbers. It liked slightly smaller jets afterwards from what I had in it (on my particular combo). I kept it on despite the pain of having to remove it for trans swaps, it reduced the clearance around the driveshaft and differential and I had to take out the extra 60+ lbs that it added to get the car back down closer to minimum race weight spec.
As for porting, any smoothing/clean-up should help, but don't expect big gains. Knowing what and where to "port" is always important. With any 2.xx or low 3.xx gears, you need torque and air velocity.
If you like to experiment and can tune a carb, also consider trying straight 4 hole or tapered 4 hole spacers . I also tried a Stealth once and found that it ran better with jets at least 2 numbers higher on the deeper plenum side (or 2 #'s lower on the shallow side, or split the difference between the two, depending how you're jetted now). But for my particular race combo, it still wasn't faster than the single plane Holley Strip Dominator I run.
