1986 Chevrolet Truck shut off while driving

I'd make sure the battery has a full charge. I'd also check the charging system. Some of the national brand auto stores will check these issues.

If the headlamps work, turn on the headlamps, turn key to crank, any reaction from headlamps? Do they dim, do they go out, Do they stay bright no reaction. If the latter is the answer, then check for open circuit to the starter.

You could use a test lite at circuit for fuses for voltage.

Just to add, some fuses go hot with the key on, some are hot all the time. Check for voltage, both ways. You may have to check voltage to and from the ignition switch and or fusible links.

You may need to check battery voltage and ground connections.