Missed that this was a truck app. W that kind of room you could probably use 4 muffs or 2 w larger cases plus some resonators/non restricted glasspacks of some type and be able get it quiet enough to suit your tastes as an X pipe MAY be difficult to practically achieve on a truck. I'd for sure go w 3" mandrel and make sure the tailpipes go all the way past the rear bumper

If its a stock-ish 440 even a compression bent 2.5" system will be fine