More than likely your flows are 2.5 and that is the same size piping you should use. A new 2.5 Flowmaster cat back system is only $250. You should also replace your stock H-pipe with an aftermarket one because that will still be a choke point. Exhaust cat-backs dont really give you any power (mabey 2-3 hp), they just give the car a better tone. You dont want to go 3" unless you are planning on making BIG power, like 500+ RWHP. You use a pipe that big on a mild tuned Stang and you will loose low end power.
just my .02
Since you already have an Magnaflow X Pipe, I would look in to a Magnaflow Catback. You would pick up a little power but nothing you would really feel but it would sound alot better.