While europeans tend to celebrate engines/cars designed for the rich, Americans tend to celebrate engines/cars designed for everyone. Accounting for inflation, the car those engines came in (the 1969 Buick GS 400) sold for ~$30k in today's dollars. For comparison a similarly powerful 1969 Aston Martin DBS V8 sold for the equivalent of $126k today.
If gas prices in the US were like the rest of the world, only rich people could afford to drive a car with a 400 cubic inch engine.
Back then, one horsepower per cubic inch was a worthwhile benchmark, something automakers bragged about.