One thing to watch with that design is that the gantry will need to be shimmed on one side between the gantry and side plates because you have trapped the long X axis rails between the sides of the bed. That is why most people put the rails on top of the profile as the tolerance can then be taken up by slightly over sized holes in the side mounting plate.
In other words the gantry length will have to be cut very accurately so the side plates are a snug fit into the rail carriages