So I have been trying to decipher what you constructed. The ‘random float’ numbers I now know effect the construction although I’m not sure about the ‘>’ tab above them.
When you say about deciding which cube to spawn depending on the X + Y parameters. I can only guess something has to happen where the green wires are?
The only way I have gotten a different cube in so far is like the image below. Although that fills all the gaps back in again 
