Thanks. Mine measures 2.75 inches. But a replacement at Advance measures about 2.5.
I looked at a friends 4 cylinder and it's alternator measured 2.5. So, I think they got swapped somewhere along the line.
You might be approaching this from the wrong angle, just a thought.
I have two known 4.0 alternators, one measures 2 7/16 ( flange O.D. ), the other 2 3/4. It also seems to me they have different size flanges, the sheaves are also built differently. They are both Motorcraft but from different years
I have no ideas as to finding a better method tho :/