Try connecting a known good battery ground cable directly to one of the starter attaching bolts. The original ground path has several connections, and a loose or dirty connection at any of them will cause the problem you're having. I make my own 6V cables from #0 arc welder wire. Do the headlights, horn, heater blower, and other electrical parts work?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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