Adding a new outlet in a old house
8-3 copper is totally inappropriate for a normal branch circuit in the US. What kind of store has only 8-3 ?! Where are you located? Is this single phase two leg 240 &amp; 120 V as in the US or 240 V single leg as in the UK?

For one thing 8 AWG  will not fit on the screws of ordinary receptacles sold in the US. #10 AWG will barely fit on receptacles at least in aluminum. I have 10-2 + gnd aluminum in my original 20-A cicuits. The 20-A circuits I added are all in copper 12-2 + gnd.   

The most you should use in copper is 12-2 + ground  (12-3 + gnd to feed a split receptacle from two different breakers on different legs). If this is a 15-A circuit use either 14-2 + gnd or 12-2 + gnd, but the former is a lot easier to work with. If a 20-A circuit, then you must use 12-2 + gnd.