[RockHQ]

63 amps if the fan has separate blades, 78 amps if the fan is a disc type fan, both are SI series alternators. Now this may be strange but your truck could've come with either from the factory, regardless of the dual battery option.

Btw your thinking is skewed in that glow plugs do not place an especially heavy load on the battery. It's the starter motor.

Because of the long cold winter over here, diesel engines are a big income source for shops and that's why i've had to deal with these engines in cold weather a lot more than i care for.

The battery leads GM uses in our trucks are a big problem. The passenger side battery's positive cable ends flatten out after being opened/tightened a few times. The rubber insulation covering the outside diameter of the cable ends hardens up. The cable ends are a sandwitch design with several different metals all connected as an electrochemical pair which causes severe oxidation.
The cable ends are too thin and too deep inside the insulating rubber.

Combine all of that and you get what i've had to witness, and deal with both in my own vehicles as well as customers, the passenger side battery positive terminal gets hot enough to melt the cable insulation during cold starting of a 6.2/6.5 diesel engine. The 6.2/6.5 liter engine is really, really difficult to turn at temperatures way below freezing point. Not only that but it needs to be spun at a steady 300 or so rpm before it will stay running. The moment when the starter solenoid plunger engages the main current bridge but the anchor isn't actually turning yet is more or less a direct short where amp draw can be in excess of 600 amps easily.

Adding a third battery is a-ok but do it in a way to get more juice to the starter!