First, do you have the headers, or do you have a particular brand in mind? It kinda depends on how the collector terminates, as to what pipes you need, and anything you want to do will have to be fabricated.
I have got three sets of "sorta" fenderwell headers. I have the very old hooker adjustables that only have the rear pipe through the wheelwell, and a set of Marco customs for a 440/a-body that have two on one side , and three on the other going through the wheelwell, but both of those have the collector underneath the car in a normal location.
My third set is a full fenderwell header in a tube chassis car, (no fenderwells at all) coated, with the collectors continued down parallel to the floor. From there, a reducer from 3 1/2 to three, a small angle pipe, then a long quiet magnaflow with offset inlet/center out, tucked just inside the rocker panel, then a turnout in front of the rear tires.
What annoys me about fenderwell headers is the sound of each bank of exhaust is separated from the other, and you kind of get that motorboat sound that i dont care for.
I have seen cars with enough ground clearance (gasser style) plumb the pipe back under the car inside the frame rails, and then you can fab an exhaust in a normal fashion.
