It's like this. You have a truck that has the same frame, same axles, same powertrain, same brakes as the same year 3500. If you add more springs, sway bar, maybe shocks, and tires if they aren't already up to it, you WILL upgrade the capacity. There are people here who always say nothing you ever do to a truck increases its capacity above the original stock rating.
That's as ridiculous as saying doing an addition to your house won't add capacity for a bigger family.
A truck is an assemblage of parts. If you assemble the same (or better!) parts, you have what you have, regardless of whether the factory screwed them together or you did.
P.S. all the weights you've listed do seem unrealistically low.
Never believe the spec, put them on a scale somewhere. Trucks and campers pretty much always weigh more than the sticker claims. Those weights are without paint, without bulbs in the tail lights, and with the interior full of helium.