The biggest standard car engine of my life was the 500 cubic inch or 8.2-liter engine in the Cadillac Eldorado with 400 horsepower which made it a true monster engine for the era back in the 70's.
that was back in 1970, and then they started to put smog crap and by 1976 it only had 275 horsepower. 
Didn’t they also change from gross to net horsepower at that time?
The pollution equipment of the time required a lot of changes to engines in order to meet the Federal government rules and regulations. They also went to unleaded gas which lowered gasoline octane quite a bit. Car engines in the late 60's and early 70's had much higher compression than those that came out by the mid 70's, so they simply had less horsepower. When I worked part time at the Esso/Exxon gas station from 1972 to 1975, at first Esso Extra was at 105 octane, Esso Plus was at 100 octane and Esso regular was at 95 octane. Then it became Exxon, and by 1975, Exxon Extra unleaded was at 93 octane, Exxon Plus was at 90 octane and Exxon Regular was at 87 octane. Later on, engines started to have electronics and carburetors were replaced by fuel injection and some of the old pollution equipment was modernized, and engines have gotten much more efficient and smaller but more powerful.