When Jaguar got hold of Daimler they had a great 2.5 V8 engine that Edward Turner had recently designed and had been putting in those little Darts. They discovered it fitted nicely in the MK II body shell and had the added bonus of being pokier than the entry level 2.4 XK unit without out-shining the larger power plants.
Being designed by Turner, it shares at least 8 parts with the Triumph T-120 Bonneville motorcycle: Its pistons! The V8-250 used domed motorcycle pistons to achieve its compression ratio.
It's lighter-weight than the XK-engined MK II so gets better fuel economy and handles better, although lacks the performance of the 3.4 and 3.8. It had the option of a 4 speed manual gearbox with overdrive, or a 3 speed auto-box, which is the same 3-speed transmission unit that goes in the Morris Oxford! The valve-seats can withstand unleaded fuel, although there's a debate on whether the valves themselves can take it. It has twin-SU carburettors and disk-brakes on all wheels.