What engine size is in your f250?
Here is vacuum diagram:
Another expert who has been with Ford since 1974 let me know that since 1976 all vehicle made in US or for US have a catalyst system. IF yours is missing, then it was removed and therefore no longer a factory system.
Is your vehicle in the US?
If yours is US made, then the diagram I supplied is the correct diagram for a 1978 Ford F250 with 460 engine. From the factory it came with a converter and Egr system. If they are not on the vehicle, then they have been removed. There are no vacuum diagrams available for altered or tampered with systems.
If you are not satisfied with answer you are not required to accept.