Well, my record was Cold Lake, Alberta to Brize Norton (4303 nautical miles). It took us 13 hours and 30 minutes. I freely admit that we only had a 25,000 lbs load down the back.
My heaviest load was a complete building (right down to the toilet seats and a flag pole) from St Johns, Newfoundland up to Goose Bay, Labrador for the Canadian Department of Transport. (It was the only way to get the building there for the sea never did unfreeze that year). The weight of the building was just under 78,000 lbs.
The Belfast was never designed to carry general freight over long distances. That was what the CL-44 did. It was designed to move indivisible loads like two Puma helicopters or a Sea King. The 80 foot by 12 foot freight bay was the largest available at the time apart from the C-5A. There were no Antonov 124s available for hire in those days.
Most people who were critical of the Belfast never set foot on the bottom rung of the spiral staircase!