. Just having a thought here but thinking if it would be possible to squeeze an extra bit of Y travel out of reworking the frame, flipping the rails over on to the bed and having the bed move. I was also thinking this would open up the opportunity to have a bit of cross support under the bed to firm things up a little and help prevent sagging..
I think I remember you saying you was going to draw it out in CAD !! As that would be the best way for a feasibility study