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When connecting your battery, connect the positive (red) first and the negative (black) second. When removing the battery, disconnect the negative (black) first and the positive (red) second. Been doing this for nearly 30 years. ALWAYS connect the positive post first.
Attach the tachometer ground wire to an engine ground of the car. This need not be directly on the battery. Much of the car frame is grounded to the battery by sturdy wires. Trace those wires to find a suitable attachment point. Attach the tachometer pickup wire.