i actually spoke to a few people about how to mount it.
story: my car when i received it, the valance was completely wrong it was mounted flat and was crooked
somebody had used all the drain holes thinking they were mounts.
the plastic was damaged so i bought another re-production to replace the reproduction that was on mine.
when talking to my mustang club where we have ford techs from the 70s that used to dealer install these.
they told me to use the center drain hole to start the alignment and that was what ford had recommended i guess for speed of assembly for the dealers.
now they told me when i use that center drain hole make sure i push the spoiler back so the front was just touching the center lip on the lower valance, they told me not to worry as i tightened it down starting from the center it would not buckle over the lip.
so mine is sitting just a hair on top of the center lip at the back of the spoiler center.
they then told me to lift the sides up and place marks if i remember 3.5"(might be wrong would have to look at my notes) up from the body line on the valance and mark both sides evenly. then i was told to loosely attach the sides so that the rake / was maintained up the side of valance, hold the valance starting from the center and work out evenly drilling holes and putting a screw in.
worked really well and i thought looked great compared to what i had before.
that was how it was done on 2 dealerships in NY in the 70s. i've since learned there are countless methods for doing it, but they all get you in the same ballpark for installation.
Since all front spoilers were dealer installed on most mustangs(except for the boss 351) there was a great deal of variety in regards to how the front spoiler was suppose to look.
i looked for a clear picture of my car when i did the install in 2008 but can't find it