Today I mounted the muffler on the Catfish.
The first step was to cut out the exhaust hole in the fiberglass. I ended up drilling holes then filing the rest. I then mounted the fiberglass body, attached both parts of the muffler with the Vband and got a good sense for were the muffler needed to be mounted. I then drilled holes and mounted the exhaust to the floor panel.
My next step will be to take the car to a muffler fabrication shop so that I can get a custom downpipe made.