[David R Leifheit]

Go to the wrecking yard/recyclers and see if you can find one in an existing GMC or Chevy. I'd suspect you'd find one in a Chevy first, as I've seen more Chevys with them. If you are real lucky, you'll find one and it will have the inner panel as well (already drilled).

Otherwise... to find out where to drill... you could always try measuring from the centerline of the nuts to the centerline of the holes for the screws that hold the panel on.
Or try running a bolt or threaded rod into the nuts, put some wet paint or ink on the end of the bolt/rod then fasten the panel back on and push it into the bolt head/end of the rod so the paint can mark the back of the panel. (hope that made sense)