wolf k,
I use the Spintechs too. Here's their web site:
SpinTech Performance Mufflers - Mufflers - SpinTech Performance Mufflers
I bought an off-the-shelf size from them at first; turned out to rattle because I didn't allow for movement of the mufflers while the car is moving. It's been a long time (eight years?) but I dimly remember needing at least 3/4" clearance between the muffler and anything else solid. A bit more is probably better.
When I got my second set, Spintech made them to my specs for a very reasonable extra fee, and they're still doing a great job eight years and 27K miles later.
Mine exits before the rear tires so it's still noisy, but it's a lovely sound!
Tom