The Atmel mega328p datasheet doesn't list or state an output pin impedance value in it's specification tables. They do however show graphs of pin output voltage Vs pin current draw for both current sinking (output low) and current sourcing (output high) both for 3vdc and 5vdc Vcc conditions and at several device operating tempertures, and the results appear linear. Not sure if those curves gives one the information to calculate a single 'output impedance value' or not.
One classic way I was shown to measure output impedance for a voltage source was to measure it's unloaded voltage and then load the output with an adjustable resistance and lower the resistance until the output voltage reaches 50% of the unloaded voltage, where then the load resistance equals the output impedance of the voltage source. However performing that test measurement would cause a greater current flow then the absolute maximum output pin current specification of 40ma. So in effect that might be a destructive test procedure to try and perform.
Maybe someone else can answer your question in a more straight forward manner?
Lefty