1250+ SUSTAINED EGT temps will kill your motor. 4" all the way through is considered the biggest you need to go unless you decide to go with some major power mods, which on a budge isn't gonna happen. 5" systems are more for sound, and w/o the major performance upgrades will not help (some say it could slightly hinder 140hp chip performance) Be careful though, once you catch pms you'll be spending much more then you ever wanted to!
Have fun, and welcome!