The truck you are looking at will most likely have a 3.55 rear end. If you want to tow 5500lbs reliably your first step would be to up it to 3.73. It will tow with the 3.55 BUT the 3.73 would be a better match and the truck and yourself will be happier in the long run. This upgrade will cost ~$800 at a non-dealer shop. See if the dealer will throw in this work as a contingency to the sale.
Don't take my word on the $800. Call around to get a real quote. This will give you negotiation power with the dealer.
With that said, you may find a comparable 5.0 with a 3.73 already installed for the same price the 3.7 with a gear upgrade will cost. A 5.0 with the 3.55 would be better than the 3.7 with the 3.73.
The 5.0 is a more capable truck and has a real nice exhaust tone.