As I understand it the "frame adapters" were supplied by the bed manufacturers. You might want to try and contact them if your bed manufacturer is still in business.
The cab and chassis industry standard was developed between aftermarket bed manufacturers and truck manufacturers around/over 5 decades ago. IIRC, the distance from the back of the cab to the centerline of the rear axle was set at 60 inches. This provided the opportunities for manufacturers to provide a cheaper truck without a bed and a market for a standardized bed starting out at 9 feet in length.
We had a local auto parts store that had a 70's GM straight side pick up truck with a factory 9 foot pick up bed. I believe they added the extra foot at the front of the bed. It was the only factory installed 9 foot pick up bed that I have ever seen.