I suppose I was asking the question. Which do you have? I simply don't recall what GM did during that transition, and I don't know what the equipment and RPO stickers should show to determine it. As sure as the sun rises, when you have it all figured out, an odd duck will show up. If yours looks like previous 1990's models, then it's a 400. If it looks like more modern models up to 2006, then it's an 800. It's important to the simplicity because a later model will fit the instruments and harnesses you'll need, rather than shoehorning everything. Look at the panoramic picture at the top of this page. Which grill looks like yours? The trucks on the far left and right are GMT-800 trucks, with different models mixed in between. Lil' Red is a GMT-400.