So if i want to try this i guess i would need a bigger then minimum sized case, lower power pc for this or get a mitx board that could undervolt the apu
Or does the heat radiation of a case acount for enough heatloss that it probably would actualy be posible with current energy consumption?
Radiation does contribute significantly, particularly if you're using a material with a high emissivity, but 60C is actually pretty low for aquarium PC's under constant load. I think your figure is overly optimistic and that you'll need a much bigger, much heavier
case to do the job. Also, you are completely SOL if you miscalculate and end up with insufficient cooling, or if you ever want to upgrade to more power-hungry components.
If I were you, I'd use something like a watercooling loop, with the radiator exhausting into the bottom of the case and the APU waterblock (or is that "oilblock"?) intaking from the top. The pump will be submerged, so you won't hear a thing if it's properly decoupled, and active circulation is an absolute necessity in submerged PC's, anyway (oil is far more viscous than air and therefore more prone to boundary layer formation). The end result will also be lighter, more compact, more reliable, and far more flexible.