Disconnect your battery and rotate it 180o. You will probably have to change the terminals on the ends of your cables. Now reconnect your battery (the negative post of the battery should now be connected to ground).

Remove the lighter gauge wire (usually brown w/green or yellow w/green on early models) from the "F" (field) terminal on the back of the generator. Now, with a length of wire, flash between the "F" terminal and a hot lead. The hot lead can be the positive post of the battery, the hot lead on a fuse box, or an "A" or "B" terminal at the regulator. Flash just long enough to see a spark. DO NOT LEAVE THIS CONNECTED, even for a few seconds! Just flash it a couple of times.

Now start the car and check to make sure the ignition light is working correctly. It should act just like it did before you started this project. The light
should go off at about 1000 RPM. If the ignition light stays on when the car is turned off IMMEDIATELY remove a battery clamp or the large wire on the generator. ("D" dynamo). Should you fail to do this, you will burn up your wiring harness or generator or both!
Some words of caution; the voltage regulator has a limited lifetime, and sometimes, if the unit is about ready to fail, the change in polarity will push it over the edge.

If the coil is original, it will be marked "CB" (Contact Breaker) and "SW" (Switch). The "SW" should now be connected to the distributor. If your coil is marked + / - , the negative terminal should be connected to the distributor.
At this point, you may want to test the other electrical items on your car. They include: The heater motor, The wiper motor, Ammeter/Voltmeter (if you have one, the connections must be reversed), The fuel pump. (Most are insensitive to polarity but some newer models and replacement pumps may require the connections to be reversed.) If you have installed a voltage inverter for radios and such, it can be removed and the radio wired for negative ground. (Your inverter can now be sold, put on a shelf to gather dust, or thrown away.)

Remove the tachometer from the instrument panel. Look at it carefully. Draw a picture of the orientation of the connections for wiring, attachment screws, note markings etc. and... most importantly... note and carefully draw a picture of the orientation of the loop of white wire running through the plastic holder. This will be reversed later in the instructions.
Now turn the tachometer over, glass face down, and locate on the sides of the tachometer, the bent tab edges of the chrome (or black) bezel. Use a very fine-blade screwdriver to wedge between the tachometer body and the bezel tabs. Bend them out ever-so-slightly one-at-a-time, turning and rotating the tachometer, regularly trying to rotate the bezel free from the tachometer body. Go around and around. The very least amount of bending of these tabs is best. When the bezel finally rotates free, turn it until it disengages from the metal body, remove and set aside. Carefully... very carefully, pry off/out the metal shroud and glass from the tachometer body if it did not come off with the bezel. Be really REALLY careful with this glass and shroud, as it is easy to damage either or both. Also, be careful to take care of whatever seals you find here, as replacements are not available...
Set the bezel, glass, shroud and seals aside to clean carefully before later replacement.
Once the glass has been removed, be very careful not to disturb the needle, or leave fingerprints on the face of the tachometer. Turn the tachometer face down, looking at the backside. Refer to the drawing you made earlier. Locate the 2 slotted screws holding the mechanicals of the tachometer inside the outer housing. Unscrew them carefully, slowly, until they are almost free. Pick up the tachometer in one "soft" hand covered with a tissue to prevent fingerprinting-about-to-happen, and complete removing the screws. The tachometer "guts" will fall out into your tissue-covered hand, hopefully, and not onto the floor.
Look carefully at the internal wiring connections. Look for the small 1/2 inch long brown, striped, diode. It is connected to the metal tab that provided electrical power into the tachometer. Nearby, look for the small green wire, soldered to a metal lug that connects to the metal internal frame of the tachometer, providing a ground. Unsolder these two connections, reverse the connections and solder in place. Do not use any more solder if you can avoid it. Just heat, remove, swap/replace, let cool. Now the small brown diode connects to the ground lug, and the thin green wire connects to the "power in" lug. This is now correctly wired for "Negative Earth", as is the picture below. (SEE PHOTOS AT BOTTOM)

Take the time to carefully dust off the tachometer face (a small artist's paintbrush is ideal for this). Replace the "tachometer guts" into the metal housing, taking care that any rubber shrouds for the marker lights are in position as the "tachometer guts" are lowered into position. Align the screw holes, and replace the screws. Clean the glass, shroud and bezel, and replace. CAREFULLY re-bend the bezel tabs to grip the tachometer body.
Look carefully at the loop of wire that runs through a plastic holder on the back of the tachometer. This must be reversed, so that electricity "flows" the other direction. You can... follow the wires until you find connections and reverse them at that point. Or... you can carefully un-thread the white wire from the plastic loop holder, and re-thread it correctly, reversed "flow".
Attach a small label to the backside of the tachometer housing stating the "modification to negative earth", Date it and initial it.
Replace the tachometer into the instrument panel, re-attaching all wiring.