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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
The stock injector size is 19lb. You shouldnt need to upgrade them unless you exceed their power limit which it right around 300hp; give or take a few. And if you were to upgrade 24lb injectors is the next step up.
If you adjust your fuel pressure form 39 psi (stock) to 60 psi, then the 19 lb injectors are good to around 330 hp. 24 lb injectors can feed a 400 hp engine at 39 psi, so this would be plenty.