He is talking about manuals, not auto. I have never seen a 10 spline 4 speed for the 4 cyl mustang. Do you have a picture of the trans? The T5 5speed is a 10 spline. There are a couple versions of the cast iron case German 4 speed, but they all have a 23 spline input.
1991 Mustang LX 2.3L 5 Speed convert
1989 Ranger 4X4 2.5L 5 Speed solid Dana 30 front axle, 4.5" suspension lift, 3" body. ( blown up....)
Entire turbo kit waiting to be used again.