Those figure is sadly completely fictional. There are and were no wels of this size. Those "often quoted" sizes are extremely obscure and with 100% only big fish stories, and the dates given for them, are completely ridiculous. The largest confirmed wels not even reached 300 cm (only 2,78 m at 144 kg), what is already monstrous. The heavies confirmed one weighed 152 kg, but was comparably short, "only" 2,52 m.
This drawings looks nice, but is based on wrong dates.