Mass air is more adaptable and tuneable. The 86-88 guys will tell you the Speed density system can work fine, and may pump a few ponies more than a stock Mass Air system, but the Speed Density is pre-programmed for a known setup and will eventually give you problems as you deviate from the known values.
If you are converting a Mustang, you can swap in the engine harness and computer from a FI mustang, just make sure you get the complete system from the same car as Ford changed the setup each year, and sometimes during the year. For a non Mustang swap, stick with a universal kit like Ron Francis or FMS etc..
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