The wing i have had on the car since 1980 has the pedestals 31" center to center, the re-productions from WWWC have the spacing of 35-5/8". I bought mine on new years eve 79 from a friend, never have known what it came off. As near as I can tell the 31" is from a 69 and the 35-5/8" is for a 70. Can anyone confirm? 
True. 31" pedestal spacing is the 1969 Cougar Eliminator rear spoiler. The pedestal plates should also be rivetted into the spoiler wing, and the pedestals should be solid - i.e. no hollow space on the underside (the '70 pedestals had this hollow casting style). The wing itself should be hollow and feel fairly light. (someone will correct me if I’m mistaken on any of this)
Hope that helps!
Good pic here of the pedestal and mount style.
http://www.boss302.com/smf/index.php?topic=69824.msg461765#msg461765
Anyone know if anybody is re-producing the 69? WCCC only has the 70.
Not that I’m aware of. All the repo has the wider 70 mount.
I have a rear spoiler with the 31" spacing. I won it on fleabay a few years ago for my Mustang- but just discovered the Mustang pedestals are 35" spacing. It appears to be old aftermarket - smooth fiberglass with screws, not rivets for the mounts. I can post pictures if interested.
It could be that someone drilled out the rivets and replaced them with screws. Anything is possible with a 45yo part that has changed hands many times over the years 
Perhaps its the camera angle, but the spacing looks closer to 35" than 31" in the final picture with the tape measure, to me.
I’m interested if still available…
Brian.
I did more measuring this weekend. My spoiler measures 12" from the end to the outside pedestal, and 32" between the pedestals, so it’s not a '69. The hunt continues.
'71-'73 Mustangs used a 33" pedestal spacing (centerline to centerline) which would equal ~32" between the pedestals. IIRC, some Mopar applications used the 33" spacing as well.