Bidule
Paper airplanes don't count - as others have pointed out, the A380F was never built.
To get your quoted 151 ton payload, Airbus was going in increase the MZFW by 41 tons, while decreasing the empty weight by a further 26 tons (I wonder if they could have achieved that empty weight number - once they add all the extra structure and main deck doors for a freighter it generally doesn't make all that much difference in the empty weight compared to the passenger version). The passenger A380 can only carry about 84 tons of payload - less than the 777 P2F.
As I previously posted, the 747 was designed from day one to make a good freighter - the A380 wasn't. If Airbus had gone for the higher MZFW for the passenger A380, a P2F mod might have worked, but they didn't and it's too late now. 20-20 hindsight and all that...