Cutting and welding the stock pipes implies reusing a significant length... Doesn't sound like a "catback" exhaust to me at all! If I am wrong, please post pics. Another idea would be for you to sell the entire setup to someone willing to exchange an uncut stock exhaust. That way, you could guarantee a good fit and easy installation.

That said; I don't want to volunteer, but I am curious enough to probe you for more information. So, is it a cat back setup in the true sense of the term?