Where do you find the rpm sensor on a 1991 f super duty 7.3 non turbo?

On a 1991 Ford F-Super Duty 7.3L non-turbo, the RPM sensor (more accurately called the Distributor Pickup Coil in this context) is located inside the distributor. There isn't a separate sensor like you'd find on later electronic fuel injection systems. You'll need to remove the distributor cap and rotor to access it. It's a component within the distributor itself, not something bolted onto the outside.