[68C15]

first off I have to say WOW! is that an engine or a trophy? one sweet looking engine bay.
I would go with a 6 G wire & run it to the batt stud on starter. ditch the factory amp guage. put in a voltmeter running from the ign on hot wire at switch & other end of gage to ground. that way the gage shows system voltage. solder all connections as with that level of current any resistance will create a lot of heat & possible start a fire.