Generally, the trick is to run the pipes a la GM F-body, which means going over the axle and to the outside of the leaf springs at the same time. That can seriously limit your rear tire size, of course, depending on pipe size and where it comes to the outside of the frame rail. Another trick is to loosen the fuel tank straps and slide the fuel tank over a notch. The last guy I knew that did that made a spacer for his filler neck (at the quarter panel) to maintain proper depth in the tank, but I don't know that it's necessary. Regardless, that allows a 2.5" pipe to be routed along that side without issue while leaving room on the other side for a matching pipe. In the case of my Imperial, the plan is to cut the tank lengthwise and section it a few inches. It's total overkill but I'm goofy about exhaust. I plan to run small resonators back there. They won't clear any other way that I can see.
Another option is to go the Challenger T/A route and dump ahead of the rear wheels. That requires a single-reverse-flow muffler to do correctly, but you're not limited to expensive repro mufflers. Flowmaster, DynoMax and others make suitable high-perf mufflers listed for late-model Corvettes with the inlet and outlet on the same end. "Reproduction" upgraded T/A side-exit tailpipes are available in larger-than-original sizes, too. Cheap? No, but they give you a shiny chrome tip and solve the tailpipe issue with no fuel-tank or tire-size dilemma, and they look pretty dang cool on an F-body.
As far as getting "that sound" on a Slant Six, you'll need a split header, which is available from Clifford Performance for $400 (that's what the Duster in the video has). You can also look up Doug Dutra, who does factory-manifold splitting as well as making brand-new dual-exhaust cast manifolds for $225, but they require a heat-shielded mini starter and one cast-in locator tab used for factory machining to be cut off the block. They also aren't going to sound as racy as the Clifford long-tube header arrangement. Dutra's method of splitting the factory manifold keeps the under-carb heat riser, which is great for cold starts but awful for performance. The other options lack manifold heat whatsoever. Whether or not the trade-off is worth it, only you can decide.
One thing to keep in mind: the smaller the diameter of pipe you use, the less it will sound like a V8. Smaller-diameter pipe cancels out lower frequencies and accentuates the higher (blattier) sounds. The flip side is that with larger pipes, you lose bottom-end torque. You haven't got much to fall back on with a Slant Six; they're not exactly rev-happy engines.
I hate to refer you to another board, but
this short thread at SlantSix.org has a pretty good discussion about exhaust options.
