Stick to the standard recommended spark plug at the recommended gap. Although it will not make much difference a little wide or a little narrow, it is always best to stick to the correct recommended gap. Here is how to adjust timing on most older cars including all the Thunderbirds. Find a vacuum source that comes directly from the intake manifold. Hook up a vacuum gage. Loosen the distributor hold down bolt - it will take a 1/2 inch wrench. Start the motor and let it warm to normal operating temperatures. Your vacuum gage should indicate a steady vacuum reading. If it is not steady it can indicate a minor or major problem with your motor which should be investigated and corrected. If you observe a little fluctuation in the vacuum readings this generally indicates a mixture that is not set correctly. There are two mixture screws that can be adjusted to smooth out the idle. Start with the screws about 1 and 1/2 turns open from the fully closed position. Watch the vacuum gage to make sure it is constant. If not back out screw 1/4 of turn. This is a trial and error method. You may have to adjust the idle screws several times to get a constant vacuum reading. If you can't then you have some other problems that will take more than turning a screwdriver. If your motor is in good general condition you should see between 15 and 18 inches of vacuum at the slowest possible idle speed. Note that vacuum reading. Now carefully rotate the distributor for a maximum vacuum reading. Again all of this is done with the motor idling at the slowest possible RPMs. Let's say for the sake of this example you see 18 inches of constant vacuum on your gauge. In the "old" days when you could get 100 octane fuel which these cars were designed to run on you would be done. All you would need to do is tighten the distributor bolt and set the idle to recommended speed. But we are in the days of 92 maximum octane so I guarantee your car will ping at the setting you just made. Back off the vacuum about 1 inch by rotating the distributor counter clockwise. Again this is trial and error and you will have to drive you car to determine if you car is still pinging. If it is you may have to reduce the vacuum more. If it makes you feel better you can now put your timing light on to see how close the timing marks might be. Usually they are not but it doesn't matter. You have set your timing to the current condition of your motor and available fuels. Ken Harkema