it can be used as a "muffler", i did on my truck
but you'd want to judge the length for yourself, the shorter a glasspack is the louder it will be.

so i'd get the longest one you could that will fit from the catback connector to the first bend

also if your going with stock exhaust than your id and od should be 2.5" but i couldn't really tell you as i never had a stock 3800 exhaust for very long