Different code books give tolerances for different areas. As Vern stated, it really does depend on the framing conditions such as distance of span of the ceiling joists, size of joists, etc. The weight of the 3/4" "flooring" material should be factored as well. 1250 lbs. in a 4'x4' area is a bit much at any rate so spreading them out over the attic is the safest bet. Even if they don't crack the joists the weight can casue the ceiling to bow over time. Try to concentrate the load over bearing framing such as beams and if at all possible, walls. Most homes are not engineered for storage in the attic these days so I'd be fairly confident you need to do something else to store these boxes.
Todd's Home Services
San Antonio, TX