Yes, It can be tight squeeze, but it can be done. I have 2-3.5 inch WD 6TB drives presently installed in my EON. I had flipped my fan so the air is being pulled up through the system. I performed the flip before really doing anything on my EON.
If you read a post that I had made on 29 Jan 'EON Hardware Setup Completed - Observation" on how to install the HDD’s using of all things, duct tape.
I hope this helps.