Charging system and other wiring change help needed

Your pdf link did not work. You can use those diagrams for "some stuff" but they tend to leave things out, like connectors. You can download the factory manual free over at MyMopar

http://www.mymopar.com/index.php?pid=31

Here

http://www.mymopar.com/index.php?pid=109

So far as the ammeter the "place to start" is the MAD article which is pretty good, and explains the "whys"

http://www.madelectrical.com/electricaltech/amp-gauges.shtml

Now as to a relay for the "big red" this is a bit convoluted.

In your specific case, that is, using that MSD, there is no need for a relay, IN FACT the big red should ALWAYS be hot. The "small red" for the MSD is like a relay trigger wire. That is what should go to the ignition switch

IN FACT you can (in reality) leave the ballast in place, and simply take the original coil + wire and wire it over to the small red on the MSD. Why? Because it's a trigger wire, and draws very little current. There will be very little if any voltage drop.

If you want to remove the ballast, simply splice all the wiring that goes to it together.

Also PAY ATTENTION to the MSD wiring. DO NOT CONNECT anything at all to the coil EXCEPT the two MSD wires. DO NOT connect the old radio suppression cap or the tach to the coil