It depends on how you define custom.
You could buy aftermarket headers, high flow cats and have your H/X pipe custom made at a shop as well as over-axle pipes and mufflers or just put on new mufflers.
How indepth do you want to get with your exhaust system?
__________________ Black 2005 Mustang v6, 5 speed, IUP, EUP and Leather.
Razzi 1/4 window scoops, Mustang GT Sway Bars, Flowmaster American Thunder Single Exhaust, Clear Front Turn Signal Lenses, Demolet CAI, X-cal2 91 tune, Hurst Comp/Plus Shifter, FRPP T-Lock, Ford 3.73 gears | 
I need some help with exhausts. 
