There are some fantastic ideas in this thread, but I love the adjustable cutouts. Its a Y with a flange on the end, with three wing nuts. You get block off plates, though I would order two or three. I would have one covered, then poke holes of different sizes in the other two. See what you like best
Otherwise I would say just decide on style of muffler and dont weld it up till your sure you like it. If you put a cherry bomb on and hate it, then you can rip it off and try a magnaflow tube (I have heard they are nice) or just about any other glass pack you like, and only risk the $20-100 for the cost of the muffler, and prolly sell it for half of purcase price on CL. All in all, one of the more important mods, done in an in-expensive way. Dont settle till you love it!