I assume when you refer to AFCO struts you are reffering to the after market a-arms sold buy several what we call in Tx "DIRT TRACK Suppliers".
If you measure the stock Mustang arms you'll find a differance of 4.5" in the length of the upper &
lower"control" or"a-arms". The Mustang frt.end likes A 13" lower control arm. This makes the length of the of the upper a-arm figure out to 8.5 ". If you look at all the after-market mustang frt. ends being sold you find this varies.This is maybe everybodies secret formula-Ha Ha---actually it is what the designer wants the camber setting(tilt of the top of the wheel in &out)to do when the frt. end moves away from ride hieght.
I build my own arms and personally like to use the same lengths as the stock arm when building a street rod . When building a frt.end for a drag car I like to make the upper arm a little longer to reduce the amount of negative camber under acceleration. ON the other hand a car that will be used for heavy cornering I prefer a shorter arm which will cause the wheel inner wheel have less positive camber or stand up straight(in a hard corner)--therby reducing oversteer. In any application I use a 13" lower arm. My personal street rod is a 34'Chevy and I built "indy style "a-arms with Chrysler srcew-in ball joints and rear mounted rack & pinion. weh!nough said?