Ford used a heavy brace/mount underneath the trunk to reinforce the trunk sheetmetal directly under the rear spoiler pedalstyle mounts. I suggest getting and installing those first, then using the holes in these as templates for the holes in the trunk, by center punching through these holes and drilling a pilot hole up through the braces/mounts. The spacing is really weird, even with these, as I've had to use a rattail file to hog out the hole into a slot in order to mount the pedalstals in a manner that did not cause the spoiler to flex. Note the Ford 69 spoiler was thinner than the 70, because of the sag developing in the middle. IN 70, Ford went with a thicker shape and I believe it was reinforced internally with metal, but don't know the latter for sure. Also, although there is some debate about it here on the Forum, my research of Larry Shinoda's involvement (the one who brought the Boss 302 and the rear spoilers to the mustang) shows that he had the rear spoiler pitched up in the back, basically shadowing the angle of the trunk lip.
Yes Ford did use braces, but if you have a trunk lid like I do that originally did not have the wing the holes will not be stamped into it. My PO just mounted the wing to the sheetmetal. I then cut holes and added the brace to give it extra support. I have several pics if you'd like to see them.
IIRC, the 69 cars did not use the trunk lid braces. Since they beefed up the spoiler in 70 they had to add the braces to the underside of the trunk lid for reinforcement. This is for the stock/concourse cars. If an aftermarket spoiler is bought then the braces are still a good idea. Sorry i don't have any measurements to provide as i am still looking for an original spoiler for my mach.
I bought an original tooling? spoiler from Tony Branda back in '95 when I was restoring my Mach. It was supposed to be exactly like the originals, however when I tried to mount it in the holes that I had drilled, it didn't fit. And I used the original reinforcement brackets as templates and it was off by over an inch. Luckily the pedestals and mounting pads were just wide enough to cover the old holes. Measure 3 times, drill once!!