I think your biggest issues here are going to be packaging and heat management from the exhaust system into the body, as well as possibly modifying body sheet metal to accommodate a relocated rear muffler.
FWIW, years ago, we went through the dual exhaust debate with the V6 Taurus SHO, and over near 20 years, nobody ever demonstrated a performance improvement that was worth the hassle of trying to run two pipes down the tunnel and around the fuel tank, the Contour has similar pipe routing, and it required two forward resonators or the dual exhaust was a loud, buzzy, raspy mess.
Your desires for not excessively loud and nice rumble mean a forward center resonator and as large as possible, high-quality mufflers.
With all the issues of getting pipe around the fuel tank, I believe I'd opt for one 2.5" pipe. Done right (assembled with mandrel bends, not crush bent at a muffler shop), this will easily support a 300HP engine.
I suspect there isn't much room for a centered rear muffler unless you flatten the trunk spare tire well. At a minimum, you'll need to relocate the fuel vapor recovery canister to keep it away from exhaust heat, and I'd recommend a muffler heat shield to protect the trunk contents.
If this were me doing this, I'd opt for stock piping configuration with a split behind the fuel tank as is now, use two high-quality short case mufflers either side of the spare tire well and some mandrel bends to bring the muffler outlets inboard to a pair of center exits, but that's just me. I wouldn't be willing to hack up the body sheet metal for the tiny gains you get from screwing with the exhaust configuration. A set of good tubular headers up front will give you more gains for less effort and Dollars than all the fancy exhaust plumbing you can imagine out back.
