These systems are for headers and areisted to work with Hooker Comp. headers.
2.5 inch exhaust pipe is a good size and will handle 400+ HP.
At 3 inches, your good for 500 HP
These are general guide lines not set in stone.
What is the goal or current power level of the 5.9?
What is the intended purpose of the car?
Mostly street or track. 50/50-ish?