1966 Mustang gauges not working

considering the gauges operate at a reduced voltage , voltage drop across the neg bat cable isn't very important.
gas gauge show empty- likely the sending unit. oil pressure drops- check oil pressure with mechanical gauge. if pressure is good, the sender is bad. ammeter reading won't show much unless you put a big load on the system. - turn on high beams, wipers, and brake lights to see it change. temp gauge- ground the wire to the sender and see if the needle moves

for sure grounds are as important as power, but op says car runs. voltage drop at batt. neg is not his prob. i also have been a tech and a college instructor for many years and can still get stuck on these kinds of problems