I think you're on the right track, but need to consider a few things.
You want to keep the muffler in the "factory" position if at all possible. If you move the muffler forward (closer to the Cat), it will be closer to the cab of the truck and will resonate the sound much more. It'll also add to the number of vibrations felt (even with suspended hangers).
It wouldn't be a big deal to chop the old pipe off and weld a straight piece the same diameter of the CAT outlet. A muffler shop would do this no trouble.
The Flowmaster 40 series is an awesome choice, but I recommend getting the "classic" or non-delta flow type. This one has more low RPM rumble, and sounds a little better in my opinion.
Also know that when you get the truck back from the shop, it will go through a break-in period before the sound reaches it's normal tone. Be patient if you're worried about the initial start-up sound. It will "thicken" as the muffler gets worn in.
Have fun with it! Be sure to post often, and learn how to post pics soon because we wanna see that truck!
The Black Beast 5.0
Better to Have Horsepower and not Need it, than to Need it and not Have it!
2000 Ranger Sport - 5.0L V8 - 8.8 LS rear