well i have a 98 K1500, he's kinda right about factory dual exhaust but with 1 muffler because thats how my truck WAS but mine has TRUE dual flowmasters on it, all someone did before me was cut the stock muffler off and replace with 2 flowmasters which they put on their sides, then i put an X crossover pipe in it, now what you could do is buy 2 mufflers and 2 cats and get a muffler shop to custom make your exhaust so you don't need that part from GM, probably be cheaper to, here's a link of what he's talking about, it would basically replace the Y pipe from the exhaust manifolds to the single cat on your truck, that part basically made it almost a truck dual exhaust just still had a single muffler, that part is whats on ym truck right now so mines true duals, not the fake dual stuff look at the very bottom of the page at part number 37-5903....http://www.lmctruck.com/icatalog/cd/full.aspx?Page=109
but like i said, you don't need that, a muffler shop can custom make a whole exhaust with the 2 mufflers and 2 cats from the exhaust manifolds and back, i'd suggest to buy 2 cats and not just 1, would be better to have 2 new cats instead of 1 new cat and 1 old cat together