I dislike Flowmaster's design in creating their mufflers with baffles. I have seen dyno results of fbodies where flowmasters will make less hp than other brands by a decent amount because of the baffles. Depending on the application, they usually are not that loud when the rest of the car has stock manifolds, cats and no mandrel piping. Plus, i think they are so overplayed; it seems every v8 truck and muscle car has a Flowmaster on it.
Best way to see what will fit is to go look lol. The exhaust on my truck has an offset outlet, but the PO did put an aftermarket muffler on. You can probably fit any muffler you want in there, with just a little fab work.
AutoAnything has some Flowmasters and other brands, and their shipping is free: http://www.autoanything.com/exhausts...0A50460A1.aspx