During the 1960's and a few years in to the 70's GM in Germany Opel factory had an American designer. He apparently had some connections and were influenced by the American style. Opels from these years look very much like Chevelles in the styling.
Admiral, Diplomat and Kapitän were the big Opels, just a bit smaller than the Chevelle. Smaller cars were the Opel Rekord and Commodore that shared a lot of styling with the Chevelle but a much smaller car.
Standard engine were the 2.6 litre and later 2.8 litre straight six OHC engine, or as Opel called it CIH (Camshaft In Head).
The cool version were the Diplomat with a 283 Chevy V8, but the Admiral could also be had with V8.
There was also a special version of the Diplomat, a 2-door hardtop that had a 327 smallblock, body were made by Karmann and 347 units were made for the modell years 1965-67.
Today I saw one of them at a local meeting, a 1965 Diplomat Coupé V8. The V8's all got PowerGlide as transmission.
More info here:
http://en.wikipedia.org/wiki/Opel_Diplomat
Cool car!
Jan