Looking good. I will watch this build.
A couple of suggestions tho, from experience - Ive built one but not quite as wide!;
You will need to stiffen up your gantry sides. Mine are about 300mm top to bottom and are made of 18mm MDF, and I have noticed it flexing. Im going to add a torsion box to each side to stiffen it.
Same with your Y axis. It wont flex up and down, but it will flex front to back and it will also twist if you dont use reinforcing.
I also found that tapping MDF was unsucsessful. I ended up using cross dowels and I can get these very tight.
I like the way you used the aluminium box for your bearing plates. I used angle and am finding they are digging into the hardwood guides. Ive been redesigning mine with a flat aluminium plate to prevent this, but I like your idea. Lots of surface area.