You are throwing a curve here with the "oval hole" reference. The trunk already has an "oval" area where the trunk lid is down to one layer where you are going to need to drill your two holes on each side. As the other post correctly noted, there are underlid support type brackets that are to be used when mounting the spoiler. The bracket is beefy and keeps the lid from warping or tearing out from the weight of the spoiler. The bracket mounts with through-bolts going sidewise. There is some slight adjustability on this, so you can't just bolt it up then center punch and drill, although that will get you close.
I would first mount the upright mounting brackets to the underside of the spoiler, then install the underlid brackets, positioning the same on each side, centerpunch through the underside of the brackets - one hole per side. Tthen drill out the 2 holes, using something like a1/4 inch drill bit. Then, I would take a long skinny machine threaded screw and insert through the bottom of the trunk lid (after removing the brtackets), and use a nut to hold in place (nut on the top of the lid). AFter that, position the spoiler on the lid so that the machine screw slides right up into the underside of the support bracket. If it doesn't slide on, the holes in he lid are not correctly spaced. You could put some masking tape on the trunk lid and mark with a pencil where the hole needs to be, center punch and drill. I use a brad point bit to avoid the bit "walking" out of the marked location, particularly where the resulting hole will overlap with the one you previously drilled.
There reason for this testing of spacing is because I found that just using the brackets as a guide did not space the holes correctly (in terms of left to right spacing) for mounting the spoiler. Had to use a roiund file to enlarge the holes in the trunk lid to make it all fit.
The trunk already has an "oval" area where the trunk lid is down to one layer [/quote]
Are you referring to the '70 model? My '69 doesn't have an open oval space. My '70 did though, where you could easily mount the bracket. Here's a quick pic of the left underside of mine......notice it is solid bracing so you must drill through two layers. The black lines are some I made with a grease pencil to show approx where the spoiler studs will come through.
I didn't want to jump into this, but the 69 and 70 trunk lids are different. the 70 model received the oval cut outs as Ford tooled up for a much heavier, solid fiberglass spoiler necessitating the use of support brackets for the pedestals. These brackets were never used on the 69 models as the spoiler was a much lighter, hollow piece. You can add the supports, but if you are using the repopped hollow spoiler (no one to my knowledge makes a correct 70 spoiler), then you don't need the supports....