There's some truth in most of what has been said on this thread, but there's also a lot of misinformation.
First of all, the 1/2 ton, 3/4 ton, etc. goes back to the 1920's and 1930's and referred to what payload a truck could carry. It applied only to a complete truck, cause there would have been no way for the manufacturer to know what kind of a body would be mounted on a cab/chassis or what it would weigh. Tt least as far back as 1947, there was a GVW rating on all trucks and there remains a rating on all trucks today.
The Gross Vehicle Weight Rating on a truck refers to the total allowable weight of the truck, body, driver and passengers, fuel and other fluids. Always has.
Every truck has a Front Gross Axle Weight Rating(FGAWR) and a Rear Gross Axle Weight Rating(RGAWR). These ratings are based on the lesser of the tire, wheel, frame, axle and spring capacity. What this means is that the lightest component determines the GAWR. Putting great big tires, dual wheels, heavy duty springs and so on doesn't mean anything if there is a smaller component in the chain. Additionally, there is a brake rating that can, and often does, limit the GVWR to less that the sum of the GAWR's.
In the old days, what we call a 1/2 ton had GVWR rating of 4,400 lb. It crept up in to the 5,000's over the years and then, when the EPA decided that we needed catalytic converters on trucks with GVWR's of 6,000 lb. or more, all of the manufacturers came out with heavy 1/2 tons with 6,050 lb. GVWR so we could use regular fuel. a new GM truck today that we know as a C1500 has a 6,400 lb. GVWR.
A 3/4 ton used to have a 7,200 lb. GVWR until the EPA decided that catalytic converters were needed on trucks up to 8,500 lb. GVWR. All of a sudden there was a new truck from each manufacturer rated at 8,600 lb. GVWR. Today, the GM C2500 is rated at 9,200 lb. Ford's F250, I believe, is 8,800 Lb GVWR.
The net effect is that the old ratings have no meaning and the only way to compare capacities on trucks is to weigh the completed truck and subtract the weight from the GVWR. You'll find that in many cases, a Ford will have a higher GVWR, but because it's heavier, a lower payload than most GM trucks.
As to the body installation, the dealer has almost always handled that with an outside body manufacturer. The dealer sent the truck to a body manufacturer or distributor and sold the truck to the final user. Even today, the body people seldom take ownership of the truck.
The problem is that a lot of people don't know what they are going to carry and tend to buy a truck that is too small. Utility bodies are a prime example of this. The driver puts 5 widgets on the truck for a job, uses 4 and the fifth stays on the truck. Over a year or two, the truck gains a lot of weight and nobody can figure out why tires and brakes and the like wear out, not to mention bearings.
As you probably have figured out, I sell GM trucks for a living and have been doing this for 45 years. I try to get my customers to use GVWR's, but it's hard to break the thinking of a lifetime. Sadly, most truck salesmen have only a very basic understanding of the facts, so it's pretty easy to get bad advice.
Just as a side comment, one of you mentioned how the states regulate axle and total weights. There is what is called a bridge formula that uses the distance from the center of the front axle to the center of the rear axle to calculate the maximum allowable weight for a truck. The longer the distance, the heavier the total weight allowed, but every state also tells you what the maximum load you can have on a single or tandem axle and you have to load your truck so that both ratings are not exceeded. This will explain why you used to see boat trailers hooked onto the back of trucks and why, at least in California, dump trucks and cement mixers have a hydraulically operated rear axle that is lifted up when the truck is empty.
I get a little longwinded on this subject, so I apologize if I wasted your time
