Building a dual mITX kinda cube-ish case ain't easy. Does anyone know common measurements, like PCIE screw threads and measurements? Mostly 120, 140, 200mm fans, HDD/SSD mounts, Board mounts etc.
Well you need different mounting holes, but that's it. You should be able to design a mounting solution that fits either if you got the space.
If you have more specific questions about your case, it might make sense to open a thread about your specific topic. We're currently off-topic in respect to the original question.
Mountings are basically the same, if you have mounts for the fans the rads should mount as well. The tricky part with rads in they extend off either end for the reservoir tanks and fittings. So that's why many cases have slots instead of holes for the rad mounts in order for the rad to be shifted back or forth as needed to fit.
If you have room for 360mm rads then 280mm rads should fit too assuming the extra width isn't an issue.
Mountings are basically the same, if you have mounts for the fans the rads should mount as well. The tricky part with rads in they extend off either end for the reservoir tanks and fittings. So that's why many cases have slots instead of holes for the rad mounts in order for the rad to be shifted back or forth as needed to fit.
If you have room for 360mm rads then 280mm rads should fit too assuming the extra width isn't an issue.
If there's enough room in the case that the fittings on a rad would fit in either orientation, then it's not necessary. The slots are useful when there's limited space and the radiator will have to be shifted one way or the other to fit.
Also since most radiators have their ports on one side (cross-flow radiators are not as common) slots let you orient the radiator in either direction depending on how you want to set up your cooling loop.
It also depends on how versatile you want your case to be for mounting options. Check the following video for the Evolv ITX case for an example:
At the 2:45 and 3:00 mark you can see the top panel with a *very* flexible set of mounting holes for various fan and rad configurations.