good luck on curing the cackle. if it's a cackler, it will always be a cackler. i suggest running Mobil 1 synthetic tranny fluid, it will shift a little better and last longer. (you'll need that with the mods your planning) the cackle might quiet some with an exhaust system.
once you get done with the "typical" mods; air, chip, exhaust, etc., you'll be ready for a new turbo impeller and an ATS intake housing for the intake side of the turbo and a banks exhaust housing for the exhaust side. That'll really make it take advantage of the extra air in and out. You'll be limited to the 3" in and 3" out on the turbo and the air will literally back up on the intake side and cause your turbo to work too hard, and then it won't flow fast enough through the exhaust side and that's where the larger housing comes in for the exhaust side. After that, go propane; yeah baby, no one will be able to touch you with all that. Forgot, don't let anyone tell you that you need injectors, they'll help but not worth the cost at all. They won't really be needed until you do all of the above and maybe then a little. They're pretty good for quite a bit of power, a higher flowing pump might be in order though before that.
The 7.3 is so much fun because, like the banshee, the sky is the limit. Damn, I miss mine.
one thing my '03 does is idle rougher than shit. i mean it sounds like it has a very aggressive cam in it, whoomp, whoo, whoo, whoomp, sometimes like it might stall. But when you push the pedal, the mofo is goooonnnne. I don't want to take it in for the idle, because I know they'll run an update on the computer and change some emissions settings and all, and it won't run as hard when I get it back.