If you are looking for a 45-50 tons NET payload airplane for flying 10 hrs the best and cheapiest option is a DC10-30F.
You can get one for no more than U$D 5 to 8 millon dollars and fly 150,000 LBS NET payload to a 9 hours sector.
I know that the fuel burninng is much higher than the B767 but its load and volume is much better.