http://www.cars.com/chevrolet/silverado-1500/2004/specifications/?acode=USB40CHT277C0scroll down for truck weights/ratings . Real payload is with OPTIONS, such as a larger engine than the stock configuration. That 1400+ payload disappears fast plus the fact that the factory takes into consideration the axle capacities as well as tires. That's probably how that 600 lb number got configured by the factory.
Pay close ATTENTION to the rear axle rating listed in the link provided. Tire ratings are stamped on the tires. You need LT not P tires.
You have to add up the cost of aftermarket gadgets (which only makes the truck feel a little better and does NOT change the payload capacity) opposed to selling and buying a larger used truck with higher stock ratings.
You need to weigh your truck and see what the rear axle weight is when the truck is empty. Then you need to ask yourself, will it be safe/smart to add 2000/3000 lbs to that axle ???
Limiting factors:
Adding a camper places the weight on the rear axle.
Again very important: Look at the rear axle rating of your truck and look at the tire/wheel rating stamped on the tires.
