Take it from a former mustang owner...I had a 99 cobra for 5 years. Had 322rwhp N/A and 483rwhp with 11psi and an intercooled Novi 1000..later added a 90mm MAF so probably hit 500. You'll get the best info on corral.net.
Your 98 has the WORST flowing heads around. The best thing you can do is start with a good platform and do a 99-04 GT heads/cam swap. Thats a 60hp gain with stock heads/cams and even more if you have a good valve job done and porting. With forced induction, those heads will equal another 100-150rwhp over the ones you have. The next bad thing about your setup is the auto tranny. That torque converter and the programming in the chip that controls it is terrible. You should get a stallion torque converter and a decent chip to crisp up the shifts and raise the lockup rpms and stuff like that. Itll be like a whole different car with that TC and chip alone. Add the 99-up GT heads/cams and you'll be very happy. By far the best exhaust combo (I had 9 combos over those 5 years and was extremely happy in the end) is stock exhaust manifolds, an offroad Dr. Gas X-pipe and magnaflow catback...not magnapack..get a magnaflow setup. Its important to have an exhaust shop make a little tapered donut that acts like a gasket on the drivers side, between the factory headers and the X-Pipe's collector. The Dr. Gas was designed around shorty headers, so it doesnt quite fit right with stock manifolds and it will leak and it'll sound raspy. Everyone thinks thats just the nature of the X-Pipe. Not true. If there are pinhole leaks all over, then its raspy. So throw that tapered steel donut/ring in there when installing it, use the exhaust clamps to bolt up all the sections, and when its all happy, have an exhaust shop weld up all the seams. In the end you'll be VERY happy you spent the extra money. The Dr. Gas puts the X about a foot farther forward than everyone else's, which brings the resonant harmonics up to a higher rpm. Its an incredible sound Ive never heard on any other mustang. The position of that crossover makes the power gains much higher than anyone else's x-pipe.
You do all this, and you'll have a good platform to start with forced induction. With a K&N and maybe an intake tube and march fluidamper underdrive pullies, you'll have about 250rwhp through the auto tranny. If you're going to spend the BIG bucks on turbo'ing it, then forget the X-pipe obviously. If I were you, Id do all the above and add a kenne bell twin screw blower. Forget the centrifugals with the auto tranny. With all the above I mentioned, add alky injection and a dyno tune and you'll have about 430rwhp.
Theres a guy here..forget his name..its like mark99GT or something like that...he had a 99 GT with a 72mm turbo (i think) and he ran 10's, and it was a very streetable car. The key with forced induction is not cramming more boost through bad heads. Boost is just a measurement of flow restriction between the intake plenum and cylinders. Its all about FLOW. With high flowing heads, you'll get far more air in the cylinders at a much lower boost level, and consequently, less heating of the air charge, which means you can add more timing and make alot more power. My buddy has a 97GT, same engine as yours, and an auto tranny. He added an eaton blower on an otherwise stock engine, but he has a good H-pipe and catback. With a good dyno tune he only made 288rwhp and 320rwtq. Thats at something like 9psi. Those heads simply KILL power potential.