I recommend installing the radiator and fan first. Fan should be pulling air through the radiator. Plug in the front I/O next. Route the PWR and usb3 cables as best as you can. Followed by slowly angling the PSU, with all the cables already plugged in, into place. The PSU cables will be very tightly bent. The radiator tubing will able be bent very tightly. Just make sure the tubing isn’t kinked, and no cables are inside the fan. Secure PSU. Plug the PSU cables into motherboard last. If you want to push air through the radiator out of the case, you'll need to get a 92mm fan grill to keep cables and tubing out of the fan. But space will be even tighter with the grill. Some of the excess 24pin and 8pin cables may be tucked under the motherboard. You’ll also need low profile ram in order to close the side cover.
Good luck!
Thank you very much for your answer. Will try it again. maybe i have to be more rude concerning the cable bending....
Anyway the free space on your picture looks pretty good. do you have 2x PCIE cables plugged in? Does the cable tucked under the motherboard affect anything (temperature of motherboard etc.)?
Greetings.