Anything with long tubes will necessitate custom piping. I suggest you buy the mandrel bent tubing from Josh @ Mandrel Exhaust Systems and go to town. This means you or someone you pay will need to do the welding.
- Single or dual outlets. Dual 2.5" or Single 3" piping would be more than enough for an N/A application. I picked single 3" to save weight.
- if you are planning to re-use your MAC Flowpath mufflers, I'd size the back end for the port size on your existing mufflers. If you decide to go bigger, go with something else that is also baffled.
- BTW, I know from experience that with no cats, a single 3" cherry bomb is NOT enough muffler for any car if you like your neighbors and your own sanity. It's waay too loud. Good news is that your flowpath mufflers seem to have a baffled design which will cut down significantly on the loud crack/pops. I ended up welding a 3" flowmaster after a cherry bomb that really did nothing to quiet the high pitched exhaust popping. I honestly think that without the flowmaster 40, I would have been too loud for even the race track (95db around here).
- X-bend or H-bend. The idea of crossing over the pipes is that the exhaust pulses will help even out exhaust flow and maximize exhaust velocity. I used an Y-pipe as it's an easy replacement for your 3rd cat and I have a single exhaust outlet. If you are going with a dual setup, I'd probably pick an Xpipe because of it's increased scavenging effect and the fact that it's compact enough to fit underneath the car there the 3rd cat sits.
- Whatever custom system you put together, make sure to weld flanges on the design so you don't have to unbolt an unwieldy 15' system as one piece. Other people have used slipfits but in other threads, people have complained that they eventually started leaking.
- Consider keeping your cats since your car is a DD or replace them with high-flow cats since they are probably near the end of their life anyway. It's better for the environment (aka the air YOU breathe).