Well if it were me this is what I would do. First I would pull the starter, Clean the contact surfaces on the starter and engine block. Remount starter. Install new 0 gauge or bigger ground strap from starter mounting bolt to frame. Install new negative cable (0 gauge or larger) from battery to frame. Install new positive cable from battery to starter. Make sure all conections are clean and tight. Then start your truck and drive it.

If that don't work I don't know other then trying a new starter?

Good luck!

Brian Moore
1949 3100 5 window Deluxe
"Today is better than yesterday, but not as good as tomorrow"