Paul, I've seen plenty of the nice custom aluminum radiators, they can fit stock brackets in most Chevys because of the simple chev design.
But to have one fit an Opel, I think they'd say, "Whaaaat?"
It's usually custom fab work. Usually pretty simple, too.
Your situation calls for as big a radiator you can stuff in there. As did mine, I went with a '85 Honda Accordian unit complete with twin electric fans. Huge compared to stock GT!
Electric fans are the trick.
Stay Cool!
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No Opels were harmed in the filming of this movie.
However two Mustangs, a Pinto, and a Capri were hospitalized.
One Mustang was euthanized the next morning.