" How long will the 6 volt coil run with the 12 volt system? "
Depends on how many actual hours of run time, the ambient temperature and the age of the coil.
Running it one hour a day for 3 months at 50* F is not the same as running it 8 hours a day at 95* F.
The problem is that overtime, heat melts the insulation inside the coil. Unless you leave the key on and the points are closed, coil failures rarely occur in 5 minutes.
Rather than guess at the coil resistance, just measure it. The Ford OEM 6v coil spec'd at 0.470 to 0.510 ohms. New 6v coils are about 1.5 ohms. 12v coils are around 2 to 2.25 ohms.
See tip # 30.
All you need is the oem ballast resistor and a 12v coil.