Factory shots of the tailgates up are rare, here is one from 1951, it looks like inner to outer to me.
https://www.flickr.com/photos/advance-design_parts_co/12315229183/in/album-72157640522200463/
Here is another view, inner to outer again, 1950.
https://www.flickr.com/photos/advance-design_parts_co/15568906080/in/album-72157648801943787/
Inner to outer 1947.
https://www.flickr.com/photos/advance-design_parts_co/16622282754/in/album-72157651718017649/
Inner to outer 1948.
https://www.flickr.com/photos/advance-design_parts_co/17058508639/in/album-72157651722976780/
I would say correct OEM is inner to outer but I will also add do whatever you like it is your truck.