If they're North American Flatheads I don't think you could keep even one in there long term, they're really pond fish, minimum of 700 gallons. The get monstrous in the wild - up to 5', and even if it only got 2' in an aquarium that tank wouldn't be big enough for a single specimen. Catfish are bottom dwellers and thus footprint of the tank is more important than gallonage, so the tall build of a 275 gallon oil tank is essentially the equivalent of a 100 gallon tank, since the cats will only use the bottom. Also, I think they're very competitive in the home aquarium, and several of them may quarrel. If you only plan to keep them until they outgrow your tank, I would recommend against it. The bigger a monster cat gets the harder it is to get rid of. I had to get rid of an overly aggressive Red Tail Catfish that was only 10" and it was very hard to find a proper home.
Do you have pics you could post?