These old trucks didn't have much for ground wires. But as the design advanced, the number of cables increased. It's one of those things that you can't have too many of. As the truck gets older, rust and corrosion get worse and cause problems with grounds.
My recommendation is a ground cable from the battery to the engine. You also need a 10 gauge minimum to the frame. A wire to the cab, bed and radiator support can be attached to the frame. This will get around all the rubber isolation mounts.
When connecting the grounds I use fine sandpaper to clean the rust and paint from the mating surfaces. Don't depend on the bolt threads to carry the load. Apply a dab of di-electric grease between the bared fittings and metal and bolt together. This is an important step with anything transistorized, like HEI and radio.
You can buy a tube of di-electric grease at the parts store. It's a silicone base with a filler added that conducts electricity.
