I think Ricks idea sounds fine...I do have the spoiler on...it came like that from prev. owner, but looks fine...but mine is a 69...not sure of the hole pattern for 67,68 and 69 are same....the 69 spoiler is wider, prob. could have same holes??
but not 100%...but on top of that I would have to dissasemble mine to get to the holes....
as long as you know where you want it...you can for instance put tape on the trunk lid, place the spoiler w studs on there mark the tape for the holes, drill thru the tape...should be pretty accurate...

a warning for placement of a 400 emblem on the spoiler if you do...measure , but check backside before you drill, there`s 'brace'inside the spoiler that you ont want within 1/4" of the stud , if you do , you cannot mount a nut on it...
dont ask how I know!