I really doubt you'd be able to get 6bt in there. Don't they weigh like 1000lbs also? Everything will have to be upgraded to handle the torque. Not to mention getting all the computers to work. If you really want a diesel in your XK, the best option to get it to work well would be the OM642 CRD that was available in XHs and WKs/WHs. That would be pretty pricey unless you found a wrecked one or maybe a wrecked E class or M class. The newer ones have a bigger turbo and make 455ft-lbs stock. How much power are you looking to make? With just a tune on mine, I can make 460 ft-lbs.