The problem with the 3 1/2 thdp occurs when you have a header like the atr that has a crossover the runs along the right side header and feeds the turbo.
most of the others i believe feed into the rear of the right side header like the stock ones. if you have this then it will fit no problem.
I have the atr's and there was no way it was gonna fit no matter what. I had to pay someone to cut and fabricate my thdp to swing out farther and sneak over the control arm and under the a/c box. I lost a little of my inner wheel well and on a real hard bump the arm will clunk the pipe but thats because the car is lowered too. Otherwise it came out pretty nice.