If you have a smart UPS that provides feedback to your PC then it will put your PC into a low power profile when the power goes out. This is very similar to the way a laptop conserves power when on battery. I would not worry about overdrawing the 400 watts from the UPS because that's only relevant when its on battery and it can temporarily provide much more, usually for long enough to bring the PC into low power mode.
If the battery gets low the PSU may put the PC into hibernation.
Your computer is probably drawing <300W total so there shouldn't be a problem. In fact your UPS would let you know if it is being overdrawn even plugged in so if there were to be an issue, you would already be aware of it. Your UPS should provide enough power for a proper shut down.