Ok, I am making assumptions about your knowledge and abilities based on the content of your only post here so far.
My suggestion would be find and purchase said desired Flowmaster 40 series muffler. Then set it in the bed of the truck and take them both here:
Tell them what you want, and leave, let them take care of the rest. Exhaust shops are often equipped to custom bend pipes that would otherwise be unavailable. I had a car years ago that the previous owner had removed the cats from and then bent up a custom dual system. By the time I got the car the exhaust was shot and the laws had changed so that the cats had to be there for state inspection. I took it to the Monro up the street from where I worked, with whom we had an account, and got the whole system remade with new mufflers and cats, for just a few hundred dollars. I don't remember the exact price anymore, but the two cats and mufflers I needed (I wanted to keep the duals) were like 3/4 of the bill, so labor and pipes weren't that expensive.