Alternator wiring

Have you read the MAD article? Even if you don't follow the mods, it gives a good insight into what you are up against

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

Also, before you get your mind set on the ammeter, it is unbelievably easy to replace the ammeter with a voltmeter so that "nobody" (much) notices:

Read this whole thread as it covers several dash clusters. I cannot take credit, but that's how I did the voltmeter on my 67

http://www.forabodiesonly.com/mopar/showthread.php?t=119480&highlight=ammeter+conversion

Now if you insist on keeping the ammeter, AND yours is in good shape, there are a couple of things:

I would NOT use the bulkhead connector terminals for the original battery infeed (red) and ammeter charging line (black.) I would instead run no8 or no6 through the bulkhead, either a separate grommet, or drill out the bulkhead connector and run them right through.

Also check your ammeter out carefully. The studs on some meters are not even really connected to the ammeter "guts." On some models they depend on the nuts being tight, on others they depend on a sort of riveted situation. NEITHER will guarantee a good connection under hard use.

Look up Bill Grissom on here. He advocates using a pair of high current back to back diodes to reduce current (under high current conditions) through the ammeter. This reduces load not only on the wiring going in/ out of the interior, but on the ammeter itself. It's like having a "partial" external shunt.