It looks fast too!
For people that like single exit exhausts, there is so little reason to not just have the exhaust shop build the muffler section from scratch. It's literally just a muffler and 2-90 degree mandrel bends. You could chose whatever muffler you want, and I doubt it would cost more than $150 in parts (shipped). It's the dual outlet setups that would be a huge PITA.
I'm not sure what you mean by "those", but the single Borla muffler that I used for my dingle was this one.
http://www.summitracing.com/parts/BOR-40474/
I made another adapter pipe last night to allow me to connect my other mufflers to the new mid-pipe, and swapped out the Q300 mufflers with my Borla Dingle. I really, really wish it would be possible for me to offer a full system with this Borla muffler, since it has such an awesome tone and makes more power than any other muffler I've tested (including the straight through 3" Magnaflow single)
I'm taking today off from the project, but will get back on it tomorrow. I will make an adapter pipe for the OEM mufflers, and test those along with the 3" magnaflow single that I already have.










