There was an Opel GT with a 426 at Maple Grove some years back. Rear tires were so wide they almost touched. Rolled it coming off the start so never saw what it could do.
That's the irony of it, I wonder where the other 30% went? I lived 5 miles from Opels main assembly plant in Rüsselsheim from 76 to 78 and never saw an Opel GT other than my friends, which is why it gathered a crowd everywhere it went. In the USA they offered 2 engines: 1.1 liter with 2 carbs (I had a friend in the US who had one) and the 1.9 liter with one 2 barrel. The Opel employees at Rüsselsheim thought the Americans must be terrible driver since they needed such big bumpers (74 and 75). I had a 71 Opel Rekord in Germany I bought from a German girl (interesting test drive). When I got back to the states I bought a 75 Opel 1900 sedan (same platform as the Manta) which had Bosch injection. They lost their ass on that car. The 76 Opels were made by Isuzu.
That wouldn't be a bad one to try; - it might be a bit wide though, which is why I thought Nailhead. However, the weight to power factor would be better, the car would probably handle like a dream. It would probably be more reliable than the 4 cylinder Opel "Klapperkasten" they would have thrown in there, which is the same engine (if I'm not mistaken) that wound up in the Vega.