67 GT500 single or dual belt ??

No A/C and no smog would be a duel alt pulley and belts. Many 67 GT50's have had the waterpump ,crank and P/S pulley's changed . In this case the crank pulley changed to the wrong one will not allow the correct belts to be used. With the correct pulleys and the correct original belts (Marti belts are the closest modern equivalent ) the adjustment will be mid way in the bracket regardless of what item you are trying to adjust on the engine. Do you know which dampener and crank pulley is correct for your engine?If you don't you are working blind. The correct damper will not accept many alternate pulleys .The right crank pulley will not fit the wrong dampener correctly.