There likely was no “shielding” in a truck of that era, just simply the truck was made of metal as was the chassis of the radio, bolt 'em together and you’ve got a reasonable ground.
But, I do know from experience that there are items on a pickup truck that can produce radio interference especially when worn. A worn distributor is a spark gap transmitter, as I learned when I installed a mobile radio in my S10. The audio on my radio got a lot better after a good service of the ignition system.