There's this thing called the "Displacement Tax Threshold" throughout Europe from long before the Oil Crises. It is three liters in France, Germany, Spain, The Netherlands, Austria and Sweden, two liters in Italy, Portugal, Greece, Belgium, Switzerland, Liechtenstein, and Denmark. Only Norway, Iceland, and Finland lacked them. Even in the relatively liberal United Kingdom, there was a threshold of four liters, which is the main reason for the survival of the Jaguar AJ Inline 6. The only engine in Pontiac's stable remotely capable of 400 horsepower at the time was the Super Duty 455 big block pushrod V8, which would require a redesign of the DMC-12 into something that looked more like the Vector W8, and even then, the likes of Ferarri, Lambourghini, Alpine, Mercedes, and even Rolls-Royce/Bentley, Bristol, and yes, DeTomaso would be laughing their heads off.
The real point of divergence is what if instead of being a maketeer/huckster, John Z. DeLorean had been the same sort of amateur Enthusiast/Engineer as Carrol Shelby, Smokey Yunick, Don Panoz, or Christian von Königsegg, and DeLorean Motor Corporation reflected that?