Me and a friend built a custom true dual catback exhaust a few months ago for my G35. You don't need a mandrel bender, but you will need to be very good with your measurements when cutting the piping. It was much harder than we anticipated, and I don't think we would do it again because of the amount of time it took (probably around 60 hours total). Just one angle slightly off on something and you may need to cut and redo a weld.

Also, the cost was higher than you would think too. Probably was around $350 just for materials alone, not counting the tools (cutting blades, welding wire, electric costs, etc.) used. I also only bought one muffler (Magnaflow resonated X-pipe), so if you're using multiple it will add up quickly.

Edit: Forgot to mention...The exhaust is 2.5" at the y-pipe and then transitions to 3" all the way back after it goes through the X-pipe. And this is with v1 ART pipes.

Anyway, it came out great and sounds great as well. Here are some pics/vids:







Drive-by

http://www.youtube.com/watch?v=fQMGf8X1OEM

Cold start & rev

http://www.youtube.com/watch?v=soQhY_4wuH4

 

that looks great good job, thats pretty much exactly what I want but two mufflers at the end instead of one on the middle, or two in the middle and/or two tiny ones at the end

 

I've been thinking about adding 2 Magnapack's at the end to quiet it down a little more and help with drone. I'm just hoping it won't take away the growl

Also, if you're going to do a true dual, make sure you add an X-pipe (resonated or non-resonated) for the Y-pipe to go into, or you'll lose a decent amount of torque.