Everything is pretty much a direct fit. If you go with one of the aftermarket cats which slide over the header pipe you will probably have no problem with that intermediate pipe since you will probably have some adjustment room. You will have to match the Cat ends to make sure they fit the pipes you are installing. So, I would think you would want a cat that slides over the header pipe, so you will need to get the outside dimension of the header pipe and then get a cat with that end as the inside dimension. So, I would first order the rest of the exhaust, that way you can see what the dimensions are of the intermediate pipe. Then fit trial it all in, except for the Cat, and then see what dimensions Cat will work for you. Get the proper universal Cat and then put it all together. Leave everything kind of loose behind the Cat so you can get everything where you want it. Work front to back, is what I do.