Power supply in rough automotive condition

Hi, I would like to use the UNO in the car and need to make shure the voltage peaks from the car does not kill the board (And I do not trust the connector regarding vibration). Therefore I plan not using the UNO internal voltage controller but an external with according capacitor, coil and diode suppressing the voltage peaks. A LM2940 is the voltage regulator down to 5V.

Can I feed the 5V from the 2940 directly into the 5V pins on the board or does the UNO voltage controller on the board is being overstressed?

As long as the USB port is not connected, and you have nothing connected to the DC Jack or VIN pin, then it is safe to connect an external 5v supply to the +5v pin.