I could only get aluminium if I brought a 6m length.
Maybe you could have sold the remainder on ebay for profit ?
I think another reason for putting the Z rails on the moving plate is to add stiffness to the plate, you could argue that the rails should not be used to add strength but undoubtedly they will. There are plenty of machines with the rails mounted both ways and likely it will work both ways, it's just that one way is slightly better than the other.
Machine is looking good by the way, I can see the frame is well put together but I don't see any triangulation such as diagonals. Will you be fitting adjustable feet ? or maybe your floor is flatter than mine. As Jonathan says the 2.2kw ER20 spindle would have been so much better and I suggest you should go this route to have the 1/2" capability.
I found the round rails to be a bit bulky for the Z axis but they do allow more space for the ball screw.