I just finished with that problem myself. I mounted a Saito 1.80 but would need to cut into the firewall to use the stock muffler so I bought a slimline #4006 pitts style muffler that fits beautifully. The exhaust pipes angle back a bit and drop down just enough. I did need to cut the bottom of the cowl so I could get the cowl on though it doesnt hurt it's strength at all. All in all it looks great!
Slimline has a 4006U which is made for the Goldberg Ultimate and has a 70 degree swept back on the exhaust pipes and will also work. I talked to Slimline and someday they will build a muffler specific to the DP Ultimate but I'm not sure they need to.
Good luck!