Rick,
Would the build sheet tell us?
Great idea.
So dragging out a few buildsheets it looks like the info we're focusing on is box 17 on line D. This should have some correlation with box 42 line D which is Suspension Packages -Heavy Duty
On 70 Boss 302's the box is filled with a box 17 "D" box 42 "D"
............... Did the 5/8" bar come standard on any other stang with the competition suspension package?.............
Don't have any examples of the non standard rear sway bar being installed but the buildsheets indicate the same codes (so likely same parts) being installed on the 428CJ's Mach I's and sportroofs.
70 351 Mach I's (2V's & 4V's) have box 17 marked with a "C" and a box 42 with the "D"
Boss 429 copies I have show no code in box 17 but the same "D" in 42
Just for giggles the 428CJ Cougars have an "X" in box 17 while box 42 has the now familiar "D" so same package different rear sway bar?
Sorry don't have a 70 or 71 version of the MPC. Mine are all earlier or the pdf 75
Hope this helps a little