Couple of things..... I think you may still have problems. Anything lower than 13.5v isn't really charging. In fact, I bet if you checked the voltage directly at the alternator with those loads on, you'd see something like 14.2v.
The fact that the alternator was rubber mounted would mean you're going to need an earth strap between it and the engine/Body. Secondly, if you've got some more of that cable, add a second positive line - If it can carry more current, it'll easily carry less.