Kevin and Kev:
The first three D&RGW open gons (400-402) were built for the 1964 season, the remaining three (403-405)for the 1967 season. All were built from 9600 pipe gons, which had been converted as you said from Standard Gauge Box Cars in the 50's.
According to Cinders & Smoke, the D&S converted two more 9600 pipe gons into open rider gons (411 and 412) between 1982 and 1986.
Those are the only ones I know about.