I just installed some SSAC headers (Speed-Daddy) and let me tell you they fit really nice without any major issue. Like BillBoost said, you will have to replace the flex section if you don't want the ticking sound. I left the original flex, and somehow I like the sound, sounds like those old over-powered muscle car hehe
About performance, it is a great upgrade and since you have a 2004 GTP, you will be able to drop pulley size safely until 3.6". Don't forget the cooler thermostat, the intake and the pcm upgrade, as well as colder plugs (AL104 is the way to go) Torque/Power is greatly increase will all those mods, I would say about 40HP/TQ.
Have fun, and enjoy the boost!
PS : +4 on the SD Headers!
