13.3V is fine for alternator output, so long as it dosen't drop below 13V under load then it's fine, same applies to the fuel pump, the problem with the wiring is it degrades and resistance builts up over time hence it is not able to supply the power the pump needs.

Measuring the voltage at idle is no good as that's not the problem you need to check it under load, easiest way is to run some wire into car, give it a bootful and check the voltage does not drop.

Martin