cause i have aircon i went the long way round and drilled 2 1/8 holes in the bay with a quality hole saw, i didn't have to relocate the battery but this would make things much easier...
this is from memory but should help you anyway
2 x 90deg bends from each side of the cooler
drivers side : 45degree silicone bend, then 90 degree pipe through hole into engine bay, then 45 degree bend into straight peice of pipe then 90 degree onto cut off inlet pipe (black one with the big and small vac line on it)
passenger side, 45 degree bend in wheel arch, into 90 degree pipe pops up next to battery through hole cut in the bay, then 90 degree bend into straight pipe then 90 degree bend onto SC/turbo outlet
whew
check out the pics of my install, pics tell a thousand words
2.5inch is fine if you can fit it, i reckon 2 inch works just fine for the output of the engine personall, my kit only cost about au$200 for the piping, and some of the hoses, then another au$100 for the core, but spend a fair bit after that on extra bits and peices i needed to finish it off
as always when you don't have a specific kit, it helps to have extra bits and pieces
i had a few leaks to check out the mightycarmods.com video on boost leaks, that shows my fmic system in it also
its not pretty, but it works fine!