Cut it off and bash the new one in and weld it on, if you have the one with the reduced tube size.
I would support the tub with a jack and piece of wood when removing the bolts.
Don't weld galvanised metal as its bad for you.
For what its worth they are talking about bringing in some checks for Heritage vehicles, but nothing decided as yet.
Introduction of Safety Checks for Classic Cars
Currently, vehicles over 40 years old are exempt from mandatory MOT testing...