Answering to myself 
…actually, since the Y plate has two holes and two slots for mounting the MakerSlide, the mod for either of the 40x40 solutions can be done by drilling. Keeping the two (round holes) as guides for squaring and expanding the slots by 2.65mm (towards the round holes) should suffice.
This could be done on a spare set of Y plates (total of $37) if we want to save the originals.
Here is the cad with the dimensions. The holes and slots in question can be seen bellow (top right corner).
@WillAdams good point.