Thought so. So what did the previous owner do about the charging issue that happens when the wrong cluster is in?

I don't think I have a charging issue (yet at least) but I do notice now that I have the correct guages in, when I drive with the headlights/foglights on and come down to an idle, my meter drops significantly. When I turn the tun signals or hazards on, the meter stay low and bounces in rythm with the signals. Do you think there's some damage done?

I would put the multimeter on the battery/alternator and see what it is actually charging at. You can run the car at 12v all day long and not "seem" to have an issue but it will drain your battery. If you are only charging at 11.5-12.5 you are not getting the 12v signal to the alternator that gives the excitation voltage.