As far as i know the auto and manual horsepower ratings are the same. I do believe that they either measure horsepower at the tail end of the transmission or at the flywheel, more likely the flywheel since there's no variance. the '87-'92 mustangs were rated at 225 horsepower for both autos and manuals, and in '93 Ford did something remeasuring and got 215 horsepower even though everything was identical to the previous years (for the most part, i've heard also that horsepower steadily dropped because of camshaft changes and that the mass air flow meter itself dropped horsepower by 2 or 3 because it was a slight restriction in the intake tract), so maybe Ford just decided to finally make the rating updated after several years. The ratings for 215 horsepower are from '93 to '95 when the last 5.0L mustangs were produced. If i'm wrong someone correct me, but i believe this info is correct.
------------------
'84 Mustang 5.0 LX
My car