What I just got last week for mine...
1.5" al-you-min-e-um, schedule 40 pipe.
...and some nice braided hose from ebay to make my connections...
I didn't have to go that thick on the pipe, they have tube that is cheaper and thinner but mine does run under the bus and I wanted some protection. I picked aluminum because it can only help with the cooling (even as thick as it is) and is easy to work with.
Your project is looking AWESOME by the way!