You Have bad battery connections at the battery and/or at the motor or a bad battery, Do the wiggle test, if can move by hand the cables (POS and/or NEG ) at the battery or the starter you have found the problem. Hook up a voltmeter to the battery posts and try the starter, if the voltage drops to or close to 0 you have a bad battery or a battery that needs a very good charge. The starter system on these old trucks are wired straight to the battery.
