Does it matter about the orientation compared with the floor beams, and their spacing?
I can see 667 being a good safest guess, if you have no information about that.
What would you do for a pair of 1000kg boxes that were each 3m x 0.5m?
Are the spreaders stiff enough to distribute the weight evenly right along them? If they flex, then it will cause a peakier pressure. At worst having them splaying and teetering on an inside edge would make it much worse than leaving the red box on the floor.