When I put a Powerflow cat-back on my old 8v, that was off a VR6 and didn't need a reducer but I'd say the best thing to do is measure your current pipe against the one you are wanting to put on .
from the rear of the cat it will have a small connector pipe and if this is changed to the the correct size, any exhaust will fit (mine had a 16v/vr6 exhaust fitted by using an early 16v connector)