Once you have installed the alternator, using a continuity test to confirm ground is an inferior test. Perform a voltage drop test with the engine running and alternator at full field. Good results should show no more that .2V to .4V. And, .4V would be high, IMO.

Because the alternator case is insulated with the rubber mounts, the ground wire will need to carry the same amount of current that leaves the alternator. So, if the alternator is capable of driving 140 amps then the ground will need to carry 140 amps. Secure the ground wire to an unpainted surface on the engine.