Do you have a 6 volt system? The most common cause of slow cranking on 6 volts is too-small battery cables, both hot and ground, and loose, dirty connections. Your cables need to be at least #0, about the size of your index finger or a little larger. Then, disconnect the cables at both ends and scrub the connectors thoroughly with a wire brush. Be sure the ground cable attaches to one of the starter bolts, or at least to a bellhousing attaching bolt near the starter, Forget grounding to the body like the truck did from the factory unless you've got a 100-point totally restored show winner. Then use a piece of #4 cable from the engine to the frame to ground the headlights, etc., and run another ground wire from the engine to the body. One from the frame to the bed would be a good idea, also.
Now back to cranking- - - - -If the battery terminal voltage (at the posts- - -not the cable connectors) of a fully-charged battery drops below 4.5 volts while cranking, no matter how "new" it is- - - -trash it! It's either sulfated or it doesn't have enough cranking amperage to run the starter. If the cranking voltage is in the 4.5-5.5 volt range and it's still cranking slow, fix the starter.
Jerry
