Secure the bottom of the PSU with an M3 screw reused from your Original Prusa, do not tighten fully yet. This screw has a different length depending on which Prusa model you have:

MK2(S) or MK2.5(S): length can be 18mm or 25mm (both are compatible with the psu_lower_mount).

MK3(S): 10mm long.

Verify that the PSU cables are not pinched and pass under the Y axis smooth rod.

Adjust the PSU position to be flush with the extrusion at the rear of the frame.

Fully tighten evenly, incrementally and in turn the screws of psu_lower_mount and psu_upper_mount.

Verify that the PSU cables aren't pinched and pass under the Y axis smooth rod.