Yeah, it's deserthorizons: http://www.challengertalk.com/forums/f19/got-my-sidepipes-19663/Someone here on the board has a blue Challenger with side pipes, personally I think it looks good although I am not brave enough to try it myself.
Side exit on these cars will cause problems. It's too low, you'll never get over a speed bump without dragging it. If you live where it snows the condensation from your converters producing water will ice up on your wheels, just ask ford 96 F150. The heat from the exhaust helps the tire deteriorate faster, trucks a motorhomes that have it in front of one tire and not the other, this was always the tire that shredded on a long trip, not that it went bad every trip but when one blew out it was the pipe side. It's difficult to hang the tip, to support the outside weight and the fact that you'll be putting a few thousand pounds on it every time you run into something.
They may look cool on show cars but not at all practical on a daily driver. Roush has had these on mustangs for ever, every one gets ripped off.
Sweet set up, I'd imagine it would have to be heat wrapped inside the tube or the plastic and paint would eventually discolor and meltI like the look of deserthorizons setup but I would have some concerns with how the front portions of the pipes will look after a few years of road debris being kicked up by the front tires.
Now if the OP can do a clean FG side skirth with the back end opened up for a pipe to come thru - that to me would be the best possible setup. If I am not mistaken, there is a mustang running around town here with what I just described. (see below)
Depending on the available room and layout of the side valance, I was curious if the side pipe could be contained within the valance. As it seems, that is not an option. Your thought of a rectangular "exhaust tip" is basically my other thought. The custom made tip would extend from the round pipe under the car, out past the lip, then curve up and back. Should look pretty good depending on the metal fab work.Very tight that is why a round pipe will not work transitioning from the under side going over the lip. But if I go from round underneath and then go to rectangular it should fit.
We will see.