For starters get rid of those plugs and put the NGK equivalent of what ever plug the factory put in it that came out in that year. Most likely a NGK BP5ES. Also make sure the plugs you buy have no more resistance than 0.5 Ohms from the center electrode to the tip.
Those plugs are a resistor type plug.....not what you want in that engine and gap them according to what ignition system you have in the car. As always check the voltage at the alternator as an ignition system that doesn't fire correctly doesn't give reliable info concerning the tune.
WHAT IS A RESISTOR SPARK PLUG?
Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark. It is for this reason that most racing plugs are non-resistor types.
I have seen a 0.5v drop in alternator voltage output make a perfectly good plug read change to a pig rich plug read without altering the the AFR what so ever.
Change the spark plugs to what they should be and then get back to us with more pics