Yes the flex pipe is before the main cat, but if you are only installing headers they should be a direct fit if they are the same flange, so therefore you dont have to install a flex pipe. The reason you have to install a flex pipe is because the WR y pipe does not come with one so you have to make up for that 8 inches lost.
I'm pretty sure of that, I am still yet to install mine, i have them sitting at home.
wish me luck though
