An alternate approach is to run a 2nd wire (10 guage) from the alternator through the firewall in/out of the amp guage and back to the starter relay. Also you should add an in-line fuse holder and fuse. The benefit is that now you have added a heavy wire link in
parallel to the factory harness while keeping the OE amp guage.
This will work fine for systems up to 60 Amps, but if you have many high draw components connected such as a subwoofer amplifier, electric fans you better bypass the OE factory harness.
Additionally, it is a good idea to kleen the bulkhead connectors, titen up the female tab terminal crimps and lube them up with dia-lectric grease.
Just my $0.02..