Your best bet for dual exhaust on a V6 or V8 Mustang II is to do one of the following:
1. Run both pipes down the passenger side, using a dual-inlet dual-outlet muffler (Flowmaster and Dynomax both make mufflers like this that flow quite well and sound nice).
2. Run a pipe down each side with individual mufflers and have the pipes dump either right before or right after the rear axle. This will result in the loudest in-car exhaust note, and isn't legal in all areas.
3. Run a pipe down each side with individual mufflers and have the pipes exit in front of each rear tire. This is legal in most areas, and sounds okay, but the bends are pretty tight to make this happen, meaning it can be restrictive, and the lack of room limits your muffler choices primarily to glasspacks or cheap turbo mufflers.
4. Remove the factory fuel tank and mount a fuel cell in the trunk, allowing true dual exhausts with a pipe on each side exiting the rear of the car. This is the route I chose to take, as my factory tank was already a leaker, and this car will be seeing limited driving time anyway. This is NOT the ideal choice for a daily-driver or even a car that gets driven every weekend.
__________________ 1976 Ford Mustang II Ghia: 302 with a 600cfm Edelbrock carb, Edelbrock Performer 289 intake, Dynomax Blackjack headers, 2.5" exhaust with Flowmaster Super 44s. RJS 11-gallon fuel cell, C4 tranny, chrome 16" pony wheels, fuzzy dice, brown vinyl half-top, and painted in the tackiest color ever (harvest gold, that's why I call it "The Goldenrod").
Also have a 2003 Dodge Ram (lightly modded daily driver/tow rig/office/dining room/home away from home/workshop... I call it "The Big Blue Dawg".) | 
