By the way, as you mentioned,
...i'm not truly building a concours Mustang.
...trying to keep what I can correct and am putting the reproduction decals on the car.
...a 68 Dearborn fastback so I can place the decals correctly.
Chances are some of your reproduction decals aren't all concours either (e.g., vinyl, not paper) and many decals are not reproduced correctly plus basically there is no "perfectly correct location" anyways for the spare tire decal, excepting somewhere on the right side of trunk lid & right side up. Every car deviated some from one another. Every worker on the assembly line(s) did things a bit differently than other workers.
My thoughts are, since you are NOT considering yours as "concours" and likely the decal you have isn't either, just slap it on randomly, with no perfect alignment to anything in particular such as other photos you might see, and it will look original to most people who would walk by. You can look at other photographs for a "basic idea" but try not to copy things from other images. Gets to be "cookie-cutter" and these old Mustangs were
anything but cookie cutter.
Richard