3 pin neutral saftey switch check?

Sorry I breezed right past the year. I haven't checked but it is likely that another relay in the box is interchangeable. There are some different configurations. A quick swap would at least indicate that the relay is or is not at fault.

I no longer have my paper truck manuals, after the house fire. Trying to deal with the multimegabyte online ones is a PITA. If I get time I'll try and look up the diagram.

You can still do some checking if the relay does not have the huge oversize terminals. Either buy some male and female flag terminals and make a short jumper for each pin, or buy a relay socket (take yours to be sure it fits) and then put male flags on the pigtail. Then you can plug the relay in and access the terminals, jumper across the contacts to see if the starter cranks, and to measure if the relay is getting "start" voltage from the key, etc.