I'll throw a different spin on this; call
Jaguar conversions, swaps, parts, service and advice - from John's Cars in Dallas. and look into a small block kit; then instead of the Nissan motor, get a 6.2 / 6.5 GM and swap it in. This engine bolts to wherever a small block GM can fit.
If you do a search here, there's a gentleman that swapped one of these into a Corvette. He's doing 40 mpg.
These engines can be bought for no money, they're easy to fix and better match for that heavy Jag. Between Heath Diesel and Kennedy Diesel there's enough expertise to make these engines run very well.