I would move the ground to the engine block or cylinder head at a minimum. If you have to extend the wire, make sure its the same gauge or larger. I would do this no matter if it cures the problem, or once the box is repaired. Ideal would be take them both directly to the battery. I've done what you did running main power from a Ford style solenoid, but I've never tried grounding to the body, or chassis, and wouldn't recommend anyone else do it either.
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'65 Tempest 467 3650# 11.30@120.31
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