I'll probably get it put on this weekend, but before I do, I wonder if the Y-pipe has a specific top and/or bottom? Or does this matter any?

 

I just installed a Motordyne XYZ pipe with cat and it does have an 'up'. I pulled the stock y-pipe from my G35 and layed it on the floor and placed the MD y-pipe by it and the flanges are angled so to answer your question yes there is.

 

Some pics I took during the install.

 

Yea, I noticed those flange angles after I wrote that. How's that motodyne pipe?

 

I finished the install about 11:30pm and only took it out for a couple minutes. Sounds good so far.