Ok going by dimensions and some deductions/approximations.
The 810 case is 53 cm high.
And 58 cm deep
Depending how CM measure it (H x W x D or D x W x H) going by picture I think it's deeper than it is higher.
The 5.25" drives will be around 20 cm deep into the case.
PSU is 15 cm deep.
Take roughly 5cm total off top and bottom due to casing, gaps and the feet.
Convert PSU cables to cm- power is 58cm and drive cables are 76cm.
PSU connectors are 2.5cm from the top of the PSU.
Roughly 45cm vertical distance from the PSU connectors to the top 5.25" drive bay power connector. The horizontal distance from PSU connectors to the bottom 5.25" is 24cm.
So I think I have plenty of slack cable, since the "cable area" is 45cm x 24cm. I could print out the case to scale
and see.
Right I've scaled the side photo 1:1 in PS (56cm deep by 53cm high canvas of case only) then imported that into a larger canvas, then created two lines for the cables, PSU and 5.25" drive boxes also to scale. It looks like I have loads of slack.