Ford had a nasty habit of using cheapo wire, that was a mix, not pure copper, this soon builds a resistance, as does the joint where the battery/alternator lead join.
Even a thin wire of 14/030 will help things markedly with this problem, the things that are paticularly affected are DC motors, such as windows, wipers, fan motors, 1 volt difference can make a huge difference.
The way to test is simply to check at idle with just engine running, then again with lights heater etc on, when raised to 2000rpm so long as you have a minimum of 12.8 will be fine when lights etc are on.
The on starter current could easily dip to 7/8 volts on a cold day momentarily, but this is onlya few secs and does no damage, ecu will work down to a low enough voltage.
tabetha