You might clean EVERY electrical connection & consider upgrading to a dual field mid/late 70's squareback alt. adding an auxilliary 8 gauge lead from the alt batt stud to the starter relay batt stud is a good idea(w a #12 fusible link inline) & Nacho's plan of just running 2 leads in/out from the ammeter through the firewall bypassing the bulkhead is a good idea & as said Mad's info is excellent.