If your 58 is the same as my 56, then it is possible the extra two posts are for the “55 amp generator equipment “; as it says on my wiring diagram, and how the back of my amp gauge was built. If you look at the 2 extra posts carefully you may see that they are hard wired with the other 2 posts ( again, this is if you have the same dash as I have). It does not matter which of the two posts you use on each side, or really even if you have it wired up backwards as you discovered that the gauge just reads backwards. Concerning the accuracy of the gauge, I can only reference mine that it seems to vary a lot, and sometimes shows even a slight discharge at cruise rpm. One test I have done to see if my battery is being charged (since I do not have a volt meter on the truck) is to measure the battery voltage before start up. Then after my drive, I measure it again and it should read higher than the first measurement.
Oh, and the black wire is supposed to come from the positive battery post (or starter post) and the red from the voltage regulator.
