Try and see if someone back home will crawl underneath your truck and check to see what the exhaust setup is, as well the pipe size entering the muffler. We'll need to know this before going much further.
Find that out for now, then we'll go from there.
Here's the link to the Flowmaster Super 44's for now. I'd go with the Stainless Steel personally, it's only a few bucks more and will hold up better.