You dont need to buy any additional hangers in order to install a new muffler on your car. If you buy one thats model-specific (made for 2005 mustang v6) then it will bolt up to your factory hangers that are already there.
In a lot of cases you dont even have to take both hangers off all the way to swap the muffler. Mine came off by taking off the rear one and sliding the muffler out of the front one. After that you just do the reverse to get the new muffler on.
If you decide to go duals then you WILL need an additional set of hangers for the drivers side. You'll also need a tap bit in order to thread the holes in the chassis of the car so that your hanger bolts will bolt straight onto the chassis like on the passenger side. You'll also need to trim your v6 bumper so the muffler tip will clear it.

__________________
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