To keep it simple.
1. run a wire with fusible link from battery stud at starter relay through firewall and attach to positive side of ammeter gauge
2. run a wire with a fusible link from battery stud back of alternator to battery stud of starter relay.

DONE

Make sure the fusible links are at the starter relay making them accessable.
The ammeter gauge will work but now only carries 50% of the load which is a good thing.
Optional is run a voltmeter gauge for an accurate monitor of the charging system.

Also run an electronic voltage regulator for the charging system and electronic 5 volt limiter for the gauges.
Hope this helps