It may be easier than you think to find the correct location. There should be dimples in the floor surrounding the location the jack hold down should be mounted. The posted picture you show looks to be correct for a hardtop though I can't see the dimples due to what looks like seam sealer but the proximity looks close enough if using the floor stamping as a reference.
Attached is a picture of my 67 4B LOS hardtop, which I think is the same for 68, and the jack hold down is mounted between dimples in the trunk floor. There are other dimples in the area off to the right side but this picture shows the ones that are used for a HT. I couldn't get closer because the car is stored on a lift and I was on the ladder.
Hope this helps.