The workers in the hive have likely increased in numbers as the queens comb laying space has increased. I doubt they would be overcrowded in a 450 litre barrel if they have just moved in, in comparison a 10 frame Langstroth box is about 42 litres so it’s like a 11 box Langstroth hive .
Cutting them out will be a real chore and I would advise that if you wanted to keep them to get them out ASAP as it will only get more difficult as the size of the hive/comb increases. The process will essentially be the same as the following:
Cutting into the barrel will increase complexity and removing/re-establishing larger combs will also make it more difficult. Some people will likely claim that you can trap them out, but I don’t see the nurse bees or queen leaving established large brood combs in the barrel.
Putting a bait hive for swarms is a good idea at the peak of swarm season, and may make sense if you plan to keep the bees in the barrel. If they have the full volume of the barrel available to them though, I doubt there will be swarming due to overcrowding before this season is out. That being said, these bees have obviously swarmed from somewhere, so the bait hive may pick up another swarm from the same source.