Merkx,
I just added the holes on a '64 which should be just like your '66. The reinforcement plate is in there but I wasn't able to reach it from the inside with a regular drill. I measured the hole pattern on the driver's side and duplicated it on the passenger's side. It came out real close to matching the backup plate. The backup plate didn't have any weldnuts on it so I had to make a small plate with nuts and tack it in. It's not perfect but you have to be inside the door to tell it.
