R9 280X for Seasonic M12II 520w Evo

agreed, fine.
The general rule-of-thumb for those who don't understand power supplies is that if you have the proper 6 and 8-pin PCIe connectors for the graphics card you are fine.

347W is likely well over what you'd use as the CPU uses more power and this is the max for the GPU. You may have other FANS in the system. Anyway, again that's probably a worst-case so nowhere near your max:
https://www.techpowerup.com/reviews/Gigabyte/R9_280X_OC/24.html