Nice build! I am a bit of an asus fan myself - never had their product quit on me.
It is a 60mm fan, but it does bring the vrm temps down by quite a bit
thank you for the compliment - I have responded to the same question in the comment above :)
Thank you! Originally they would have been mounted on the front, but my motherboard is too wide - with fans in front, there would be no way to plug in the motherboard 24-pin connector (on this mobo it is turned by 90 degrees). So I have moved them to the back. This is less optimal, but it makes sure that my case is pressure positive (the intake fans are ~150 rpm higher than the exhaust).
These tanks are available on EKWB website. This here is their EK-RES X3 - TUBE 250
Which GPU mount kit did you use and how much clearance do you have between the bottom of the case and the GPU? I am thinking about putting a vertical GPU kit and putting a rad underneath, but I am worried there is not enough clearance.