Should you prefer to just install the flap only, it will function less efficiently than when using the full Standard Performance Kit. With just the flap, it will open part way at part throttle, closed at idle, wide open at full throttle. 
 
How to test which flap you need: 
Before 1977: 3 inches from top to base
Mid 1977-1981: 3.5 inches from top to base