Correct me if I am wrong but...

The following Diesel engines are designs based on, not converted from, a gas engine.

International 6.9L Diesel V8: Used in Ford trucks, based on an International Gas V8 with nearly the same displacement.

Mercedes Diesels....
Most of the early Good old reliable Mercedes Diesel engines powering sedans all shared very similar features to gas engines which where available in the same cars at the same time...There where quite a few Diesels, back to the 30's with the First Diesel sedan, but I'll start with the late 70's - mid 80's since this is what we are talking about here..

Mercedes 2.4L I4 Diesel: aka, 240D shares a very similar block to a 4 Cyl. gas engine made by Mercedes.
Mercedes 3.0L I5 Diesel: aka, 300D. Like the 2.4L, it shares similar features to a 5 cyl. Mercedes gas engine.

Nissan 2.8L I6 Diesel:
Remember the Z car with that 2.8L I6 Gas engine?...

Oldsmobile 5.7L Diesel V8: Based on the Oldsmobile 350 V8 Gas engine.
Oldsmobile 4.3L Diesel V8: Based on the Olds 5.7L Diesel V8, not the Olds 260 Gas V8 like some may think.
Oldsmobile 4.3L Diesel V6: Based on the 5.7L Diesel V8 minus two cylinders, not based on the 3.8L Buick V6 or the Chevy 4.3L V6 like I have heard some say. I heard someone say Chevy 2.8L V6 once..thats not it either.

Volkswagon Diesels...
Like the Olds Engines, the VW Diesels can be converted into gas engines. The 1.5L, 1.6L I4s are very similar to the gas versions of the same time with slightly different displacement. These even have aluminum heads, just dont over heat them and the engine will run and run (I have three Diesel VW Rabbits). Early models pre mid- 1981 had problems with stretching OEM head bolts. Sounds familiar. VW upgraded from 11mm head bolts to 12mm head bolts to solve the problem..The new aluminum head TDIs remain with 12mm head bolts.

Chevy 6.2L V8 Diesel: Kind of another story. Although it does resemble a Big Block Chevrolet a little bit doesnt it.