Only you can answer that question.
For some people, 1lb is too far. For others, if you can see daylight between the frame rails and the axle tube, it's got room for more.
You're taking a truck with around 3750lbs of rated capacity on the rear axle, and throwing over 4000lbs on it... Do the math.
Sure you'll gain some of the capacity back in removing the original bed, but two healthy guys can lift a typical 8' pickup truck bed these days. Nowhere near the 900lbs of this steel flatbed you want to install.
Putting 10-ply tires on half ton trucks since aught-four.