To find the correct points gap, you must consult your owner's manual or a reputable Ford shop manual for that specific year and model Mustang. The information should be in the section covering the ignition system. The gap is typically specified in inches or millimeters.
Failing that, look for a sticker under the hood, which sometimes contains specifications including the points gap. Otherwise, contacting a Ford specialist or a classic car mechanic familiar with 1966 Mustangs would be recommended. Trying to guess could damage your ignition system.