The gold would be moved by cash-in-transit trucks which have relatively modest payload capacities of 5000-9000lbs today, a bit less in the 60s. 3 tons per truck is probably on the high end.
Was that the case in the 60s as well? I know trucks of that era had much lower capacity than today, even when comparing across class like "half-ton" trucks.
The Federal-Aid Highway Act of 1956 set the gross vehicle weight limit for trucks at 73,280lbs. I imagine trucks of the day probably at least came close to that limit?