alvis,
In 1970, the power ratings were in SAE Gross. Which, when converter into SAE Net (after 1971), comes to roughly 20% less.
Then again, drive-line losses add up to 15-16% on manuals and up to 20% in the case of autos (its an approx figure depending upon the type of drivetrain used). Hence, the rear wheel horsepower is approximately 80 - 85% of the SAE Net numbers.
In this case, the SAE Gross 450hp equates to a ballpark figure of 300rwhp.
Although, 300hp at the wheels might sound a li'l unimpressive in this day and age, lets not forget that this is a factory spec car built 41 years ago, with warranty, with all the rudimentary emission junk thrown at it and had to comply with insurance laws too.
The above is relevant to showroom floor stock ls6 chevelles only.
Now, what the Big Block Chevrolet motor CAN ACTUALLY DO - when released from the shackles of emissions, MPG, insurance and driver comfort, and equipped with the right bolt-ons - is LEGENDARY.