If you open the link at the bottom of your post, there is another link at the bottom. Here is that link

http://www.forabodiesonly.com/mopar/showthread.php?t=53282

It states to drill holes:

1.I placed the adapter on my cap. (with the cap open and on the bench)
2.Clamped it in place.
3.Drilled the 1st hole.
4.Placed a screw through the hole.
5.Lined it up and clamped it again.
6.Proceed to drill the rest of the holes.
7.Used the original screws that hold the filler neck to the Quarter panel to put it all back together.