This is the gallery that makes me confident enough to give it a go: https://imgur.com/r/sffpc/pGvkQXI There's a clear gap above the Flex, and the cable routing would still work round the shorter RPS-400 I'd expect. Plus, the issue would be the spine first, before the motherboard by the looks of it, so there's another mm or so to shave there if required.Does the K39 have 43 mm of clearance? The RPS 400 is just a bit bigger at 43 mm instead of 41 for flex atx.
I don't mind being a guinea pig.