Typical cast-iron Ford heads for the Mustang use a 14mm spark plug.  The E7TE heads use a short (0.460") reach plug while the GT-40 and GT-40P heads use a long (0.708") reach plug.  Both types are a tapered seat (no washer needed) plug with a 5/8" hex.  I'll typically run either a Motorcraft or Autolite plug.  I try to stay away from Champion and your 'rob you blind' plugs such as the Splitfire plugs.  For the Motorcraft and Autolite plugs, it basically holds true that the lower the number in a family of plugs, the colder the plug.  Colder plugs are good for high performance applications as they wick the heat away from the electrode quicker.  Hotter plugs are better for everyday driving and fuel economy. The colder plugs resist preignition and electrode erosion better.
 
 

Autolite recommends installing the plugs approximately 1/16th turn past finger tight or 7 to 15 ft-lbs.  Autolite plugs come with full-thread or part-thread.  The 10x series is full-thread

note:The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling. Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C. If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire. If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage. In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber. A projected style spark plug firing tip temperature is increased by 10°C to 20°C.