At the risk of stating the obvious, why don't you take the screws out of the gauge cluster, pull it free of the dash (while leaving it connected to the wiring harness), and look for an empty connector? If my memory is correct, those '60s clusters are surface-mount, not rear-mount like the '40s and '50s models. It shouldn't be too hard to pull it out enough to see the connections.
At the risk of stating the INCREDIBLY obvious, disconnect the battery first!
