in 1971, they also dropped compression ratios in order to handle the newer (lower octane, unleaded which had it's own changes in ratings) fuel requirements as well as coming up with the new method for calculating horsepower. So, the '71, '72, '73 engines were rated significantly lower in horsepower. Most hot rodders felt at that time that it was the beginning of the end for high performance automobiles. However, here we are in 2001 and we have the Bullitt, which puts out more raw HP than many of those supercars, lower emissions and much better mileage. The only bad news is the MUCH higher purchase prices ( the average musclecar back then went for $3000.00-$4,000.00 out the door. If you apply about a 5% per year inflation factor compounded, those same cars should sell today for roughly $15,000.00, but would more than likely sell for $30,000.00 instead).
Sorry, I got off topic a bit, but hope I shed some light on the changes in horsepower ratings over the years.