i've done an install of one of those type of mufflers before onto a really low 67 Camaro convertible.
We used a 3" dual inlet/outlet Magnaflow crossflow type muffler, Dr.Gas oval tubing, and some electric cutouts. It was the best sounding car I've ever heard.
Shoot. I just realized we never took pic of the crossflow muffler back there. We put it between the gas tank and rear end, and then the pipes exited near the 1/4 panels by the rear leaf springs.
Anyways, wish I had a sound file of it too, it was so wicked sounding.
Tyler