First, things like the A/C compressor and other OEM parts(radiator) on the TT are different sizes and dimensions. This allows for the I/C piping to be routed. In the case of the radiator, you either have to swap them or cut/weld modify the radiator for it to work(how polished are your welding skills and how hand do you have an ace-torch?)
Second. There a tons of little nuances about the TT that require "jimmy-rigging". Like the SLIGHTLY larger TT flywheel that requires you to "shim" out the starter with flat washers in order to get the starter to engage the flywheel.
There are a ton more. "I have custom mandrel bent pieces made for hardpipes and I fabricated stock recircs into BOVs and machined piping for them and had them tig welded to the hardpipes." ...AshsZ
Trust me, its not as easy as just slapping some parts on.
Don't you wish your car was as cool as mine? wait...it is? Go to hell then!