There is also two methods of ratng crank HP, gross and net. Prior to 72 or so the engines were rated gross at the crank, that is, they were tested without alternators, water pumps, air cleaners, exhaust. Then the government step in and made they use a uniform rating system that was net at the crank where the engine was tested with all the accessories that would be on the engine in the car and with stock exhaust. The big drop you saw in rated HP for the same engine in the 72 time frame was mostly due to this testing method change not the often blamed reducion in compression for emissions.
So, if you are looking at factory ratings prior to 72 and then measuring actual rear wheel HP you could see a difference approaching 40%. The net number rating to rear wheel is in the range that has been mentioned.
