Just for reference.
The Petrol V8 has a dry weight of 375#
Cummins 4BT - 745-782#
Cummins 5.9L 24V - 1150#
Cummins 5.9L 12V - 975#
Mercedes I5 - 563#
Isuzu 3.9T (4bd1-t) - 771#
200TDI - 595#
300TDI - 595#

 

You get the best mileage(they claim) converting to the 200-300TDi. But those are also the most expensive in the US. The 5.9L Cummins is up there with them however. The 200-300TDi, Mercedes OM617 engines would need at least medium duty front springs.
The others would need Heavy Duty and maybe more to support the additional weight.