If the truck runs and drives now, I would strongly recommend that your dad just enjoy some miles in it before starting any upgrades. There's nothing like experience behind the wheel to help decide what is important - and the priority of importance. Six volt systems routinely get bad mouthed as unreliable. There is some truth here. There is also truth that most old vehicles that are poorly maintained are also unreliable - regardless of voltage.
Start by tending to the basics - check or replace all the wiring, with special attention to cleaning the grounds, verify the generator function, replace or reset the points/condenser, and use a known good battery. Do that and then you'll be able to determine how reliable your ride is as it sits. Without those steps, you'll start changing out parts without understanding the base condition.
Enjoy the ride.