I just took it to a local exhaust shop, told them I wanted a DIDO 50 series with two slant cut tips 45'ed out the back and they bent all the new piping to route it how I wanted it from the cats back. All fully welded for $300 total.
You could probably takes yours to an exhaust shop and have them weld in a 50 series for not much more than the cost of the muffler itself.
Part of my thinking when I ordered the system from flowmaster was to get it all stainless steel, which comes with a full lifetime warranty. I'd imagine bringing it to a shop for a different muffler would void the warranty and I was also concerned that they don't make the 50 or 44 series (the two I was considering) in the proper diameters. I'd then want the shop to use all stainless for any piping of theirs. It may get costly I fear