Basically think 2 of everything.
2 downpipes, 2 flex joints (don't skip these), 2 cats (???), 2 resonators (don't skip these either), 2x the bends of the stock rear section, 2 mufflers (straight through), & 2 tips.
Don't forgot about making/welding in all the hanging points (minimum: 5)
Your 2 real choices for the downpipes are cutting up the stock Y-pipe (gives you the long one, but I would not use the 180 one personally) or getting an aftermarket Y (SHO or BAT - same thing) and cutting it up.
Yes it is "technically" possible to build your own downpipes from mandrel bends but it would take several bends (figure around $50 in bends) and would be very messy and easy to screw up (uneven inside!)
Also you would need to weld on spherical flanges to be able to mount them to the manifolds/headers.
Designing it is extremely simple. It's the fitment that's the hard work.
