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If you get under the truck and look at the muffler there will be a number on one of the ends. Write that number down and then come back inside and google it or try the search on the Flowmaster website. Good luck.
I think it says on the their website how long the mufflers are. I would just measure it and see what it compares to. If its really quiet in the cab than its probably a 50 series, but I'm just guessing.