There is no ignition ballast resistor in the standard 'Land Rover Discovery' harness. The original circuit will have had an ignition amplifier, fed from a pickup coil in the distributor. I don''t know the Mallory setup, but a quick google suggested a ballast resistor is required.
Note that with a ballast resistor fitted you may well read 12 volts at the coil, the voltage reduces to 8 or 9 volts when there is current flowing through the coil, such as when the contact breaker is 'made' on a conventional old style system
If a ballast resistor is fitted you should have a feed from the starter relay to short out the ballast resistor while the starter motor is 'live' so the coil gets 12 volts (will actually be less as the load of the starter motor reduces the voltage at the battery)
Do you have a model number for the Mallory distributor ?
