The only difference between a Ram 2500 & 3500 is the rear suspension. Other than that, exact same frame, engine, transmission, rear axle and brakes. The stock suspension should be good for easily another 500-750 lbs without the possibility of needing air bags or something similar.
There is absolutely no reason that with a relatively few inexpensive modifications, you can't be in/near 1T capabilities regarding payload. At least that is my plan cause I have a `18 Ram 3/4T CTD. You might not want to go that high, but I chose the CP3 engine over the CP4.2 for longevity sake.