[mr.precision]

Thanks for the replies.
Correction to original post:
Battery shows 12. 6 volts with ignition off, not 15.6 volts.
Drops to 12.45 volts when motor is running.
Battery is 6 months old and fully charged. Terminals are clean and tight.
Alternator is off another C10 and works fine.
Perhaps "new" regulator is bad..... wouldn't be the first time that has happened.
One wire alternator is great idea, but having just rewired the front of the truck I am not going to hack everything up again.
Both fuses to the battery gauge are good, as are fuse holders.
Red wire across the top of the radiator, often referred to as a "shunt wire", was replaced with 12 guage primary wire. I don't think that would affect charging rate.
Gotta wonder if the "sensing wire" to the alternator is bad, however it's all new.
Keep the ideas coming I am out of ideas myself. Thanks.