If you're staying n/a, then u/d pullies are a must have. Go with the Steeda pullies. Ive had them on my car now for 2 years and no problems with them yet. If youre going with a supercharger, then dont do them. They will throw the supercharger off.
Full length exhaust is always a good idea as well. Ide recommend Long tube headers w/ matching x-pipe and a good hi-flow catback.
For ignition work, you can get an ignition timing adjuster and advance the timing of the whole ignition input, or you can get a custom program for your car via a sport chip or dyno tune. If youre planning on doing internal motor work, Ide just go with a dyno tune.
You asked about the PI headswap. Since you have a 99+ GT, you already have the PI heads from the factory. If youre looking to still do internal work, then I recommend having your factory heads ported and polished, giving them an oversized valve job with stainless steel valves, and some competition valve springs for insurance. While youre in there, get a new set of matching cams for your combo. Send a PM to Scott and Tim. They can give you more info. than I ever could. Their user names on here are VTengines(Scott) and Cobra Killer(Tim).
Some other good people to talk to are the guys at Houston performance, Sean Hyland motorsports, and Fompkmotorsports. Here are the links to Houston performance, Sean Hyland Motorsports, and Keith at Fompkmotorsports. I hope this helps out and good luck.
www.seanhylandmotorsports.com
www.fompkmotorsports.com
www.houstonperformance.com