I did that recently and found out that the FCP header pipe's flange where it mates with the cat had elongated holes. I was just reusing the old hardware, which in my case was in good condition. The problem was that apparently the original flange did not have elongated holes, so with the new setup I needed to use washers or else the bolts would sink through the new flange holes just enough to cause a leak. So make sure you get washers even if the original hardware didn't use them.