I'm using the same incalloy heater in a 150L stainless tank (your using a stainless cask?). I made a 58 mm hole and filed the rest, found a giant nut and stuck some gasket sealant round, it all works fine. I had to fiddle with the thermostat to override it up to 100degrees.
Do you think you can bend those elements safely? Its height equals 50Litres in my boiler and thats a pain as a minimum capacity. Have you bent yours without problems?