If he wanted to keep it in the 'family' he could also use the BMW diesel from the mid-80's Continentals & Mark VIIs. In-line 6 got pretty good fuel mileage. FWIW the diesel-equipped vehicles had extra sound-proofing that included (I think) some sort of cowl baffle(?).
Same engine was also used in the 500 series BMW, as well as a ~860 vehicle production run of a super-cool 21' RV called the Vixen. Supposed 30mpg in that vehicle.
Good luck.