I've got an insert in my 2500HD. It's a regular cab gas, so from the factory it has almost 4000 lbs of payload. With the insert installed, me in the drivers seat and a full tank of gas I can still haul 2800 pounds in the bed without overloading. I installed Timbrens and it will easily haul 3500+ although I know that is overloaded. If you have a diesel crewcab that will greatly diminish your payload, although most of the extra weight is on the rear axle.
If weight is tight, look an aluminum insert. About half the weight of a steel one, and will probably add less overall weight to the truck than converting to a dumping flatbed.