Ah yes, but those were the days of "gross" horsepower ratings, not SAE net ratings. A more accurate number for hp on a 1970 GT is 82-85 (at the flywheel). But this is easily increased to 100-110 "real" hp with the proper bolt-ons.
Going even further, and modifying an Opel to produce somewhere on the order of 140-150 hp will result in a car that is VERY quick, even by today's standards.
Achieving hp levels above this reliably either requires a much larger Opel engine (2.2 or 2.4) with extensive modifications, or the installation of "power adders" such as a turbocharger, supercharger, or nitrous oxide.
With a naturally aspirated engine, the upper limit will be around 200-220 hp for a 2.0 litre racing engine, or about 230-260 for a 2.2 to 2.5 litre racing engine. But with a turbo, a "mild" streetable 2.2 litre could make 275 to 325 hp for less money than a racing 2.0 litre naturally aspirated engine could be made.
But either way, it will not be cheap. For a 2.2 litre turbo with stand-alone FI and a new turbo/intercooler/fuel system, the price will be around $6000 or so, depending on how much work you do yourself. And the brakes, suspension, tires, transmission and rear axle will all need to be upgraded. Even a Getrag 5-speed 240 will not be adequate for a strong turbo, I've had quite a few fail with non-turbo 2.4/2.5 litre street engines because of the torque they make. Assume 200 hp to be the absolute hp cap for the Getrag, after that it's bye-bye!
RallyBob


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