I am not sure what you are asking. Using any of the Model-T rear springs will lower a Model-A because the eye-to-eye dimension on a T spring is narrower. When the T spring is stretched to the Model-A perch width, that is what lowers it.
As far as a T Runabout spring being adequate, we probably first need to know what Model-A body you are installing this under. The only other comment that I can tell you is the spring's tie-bolt will locate the spring, and the perches keep it from rotating. I generally had to add weld as a filler to the spring clip bars because the T spring pack will be shorter. Once the spring clip bars are firmly tightened in the crossmember, my experiences is they don't move.