If there is no way you can find out whether it was converted to 12 volt before then you'll have to check all the components yourself. The fuel quantity, engine temperature, and oil pressure guages will need to have voltage drops connected to their power source to make them work. If you are running later manufacture Stewart Warner gauges or the like, you would be checking to see if say the fuel quantity gauge had an after market sender. Oil psi and engine temp could be direct reading gauges. The ampere meter doesn't care what voltage it is. Light bulbs would have to be checked to see if they are 12-volters, etc.
The generator could have been converted to a later 12-volt type but that will be hard for you to find out without more experienced eyes looking at what you have. Same basic thing for the regulator too. This would get you in the ball park as far as a charging system for your battery.
Basic circuits would be lights (both front & back including a brake light switch somewhere and a headlight dimmer switch), a horn (usually has a relay), windshield wiper (unless it still has a vacuum unit that works), Accessories and turn signals could also be added in.
There is a good thread on here about aftermarket wire harnesses and there are several good ones available. If you went back to six volt, you would still have to decide about updating to a voltage regulated generator or alternator set up so an original harness would be good for everything but that (they had a 3-brush adjustable current system with only a cut out). You would have to wire it in seperately but that shouldn't be too complicated if you can get a diagram for your installation and adapt as necessary depending on the Gen/Alt/Volt Reg you settle on. If you still have the ignition switch on the column drop, you could use that and a push button starter switch like original or a twist to start if none of the old stuff is present.
The original wiring harness was real basic and I'm not sure what it had for circuit protection but most of the old Fords just used a basic circuit breaker. Later harneses would at least have fuses of some type for the circuits.
Man it's just up to you, your budget, and what your vision of this thing is. Any other person may do it different but it don't belong to them.
Kerby