Krazy, we just bought a 2003 6500 (chevy) with the 3126 and we have been very happy with it so far. we paid 18k for the truck it has 135k miles right now. we weighed the truck empty and it was right around 12,500lbs. door sticker is 25,200gvwr. we have hauled 9yds wet topsoil and it handled it fine, just gotta be sensible. when working on a site, we put close to 12 yds in her and she still dumped just fine. bed is a 11 foot iirc, godwin dump. trust me you want barn doors, even on a dirt bed. we hauled alot of sod for a volleyball court and when the patches get caught on the top of the dump door it hangs up real good.
we will probably get the power turned up a bit next year b/c its a bit slower when we have our deckover and 11k lb rc100 behind her.
i will tell you its one of the best decisions we made so far, it allows us to haul much more material at once, less trips saving money. plus our dump sites often charge by truck, so we had to make 20 trips with our 1ton dump at 25 bucks per truck hauling sod before now, we did it in 3 trips at 30bucks per truck.
definitely look into getting a barn door kit or a new tailgate, especially if your going to haul brush at any time. if you dont have it you will limit yourself to keeping the load below the top of the gate. our gate is light enough for two of us to safely chain it level by pulling the top pins. but like i said we definitely want a barn door kit.