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.
good luck