Just because you have duals doesn't necessarily mean that you're gonna get more power or better sound.
I have a 2000 5.0 and I wanted true dual Borlas, but duals simply won't fit unless you:
1. Get rid of the spare tire
or
2. Run like 2inch pipes
So, instead I have a Flowmaster 3inch Y-pipe, a single Flowmaster 50 Series 3" in/out muffler, and a single 3" pipe with a 3.5" tip.
It has plenty of power, and sounds exactly how I wanted it to. It is a Limited, so I didn't get a 40 Series because I didn't want that all that droning and loud noise while crusing. The 50 Series is perfect, it sounds aggresive while accelerating, but its almost unnoticable while crusing at 70mph.