I had to install rockers on my '69 LeMans a couple of years ago, where someone had filled in all the holes, and painted over them. The post below has an entry by "nytrainer" that might be helpful. I ended up missing on my drilling a few times trying to find the holes (fortunately, those end up being covered up by the panel).
http://forums.maxperformanceinc.com/...t=rocker+panel