The gold bars being unloaded from the aeroplane would never have been loaded in that way: they would have been secured in crates or been securely bound together to stop them from moving about which could have had catastrophic consequences for the flight.
The Ford Transit van shown in the film could never have handled five tons of gold because that model had a maximum payload of only 1.75 tons, so even though it was supposed to have been armored, it was still equipped with its standard running gear which would have collapsed under the weight.
The Ford Transit van making its way to the waiting freighter still had plenty of clearance between wheels and bodywork and was easy to accelerate and stop so it was clearly empty.
As the Bristol Freighter takes off after the robbery there is a clear view through the windows and it looks suspiciously like there is no Transit van on board.