clean and tighten the cable ends, being sure the clamps make full contact on the battery posts. Make sure the starter to BellHouse contact patch is clean and tight. And the BH to block better NOT be painted.
The easiest test, is to just take a Booster cable and clamp it from the Battery Negative to the starter body, somewhere that does Not have paint on it. If it now spins normally, that points straight to a grounding issue.
Having said that,
go back to Post #13