Steel body over box-section steel frame.
It’s predecessor, the XK120 was so successful and advanced that Jaguar didn’t replace it for 6 years with the XK140. The XK140 was introduced in late 1954 and sold as a 1955 model.
Given the success of the XK120, Jaguar made only well considered changes to the XK140, and retained the overall styling. The primary visual change from the XK120 was the more substantial front and rear bumpers, with large overriders. Improved layout of the engine/gearbox assembly resulted in a forward weight shift which, along with new rack-and-pinion steering, improved the handling. Another new feature was an upgrade to modern flashing turn signals.