where to mount rear spoiler on 73 duster

I will tell you where I put mine..I have a 72 Demon without the inside trunk brackets and I think it will be just fine. Lots of metal there to keep it from damaging the trunk lid.
From the top of the trunk at the back window I came down 21 1/4 inches to the top of the first pedistal, from the side of the trunk lid I came in 12 5/8 that was with the tape flat against the side of the rear panel.
Hope this helps.
AL