The factory rating, at the time, was an average flywheel/flexplate rating and, in the case of many of Ford's performance cars, likely conservative.
The rating assigned to your car was likely near reality for the way it was measured. Tack on some accessories, a transmission, driveshaft and rear end, and you have the ratings you've read in this thread...
I would imagine, since I believe the rating was with smog equipment functioning, the removal of the smog equipment and its restriction in the exhaust, might help a bit. I remember turning a smog pump into a vac-u-pan vacuum source on my old 67 after they started smogging them...the equipment was there and functioning, maybe just not the way they intended *G*
The HP numbers you see today are "with" engine accesssories and exhaust. ie: Power Steering and Air Conditioning , where as in the 60's Ford Dyno Tested the engines with open headers and no accessories giving higher then normal numbers.
Lets say you bought a Hi-Po 289 expecting 271HP but you also got Power Steering and Air Conditioning and of course bolt on a full exhaust system. Well....you would immediately drop down to about 220HP and out the back end (HP to tires) you would be about 195-200HP.
See how fast you loose that extra 71HP.
Now....I belive all new engines are still rated for HP at the flywheel but it includes all accessories and a full exhaust system.
There was a series in "Mustangs and Fords" awhile back called "Plan for Ponies" where they took a stock A-code(4V) 289 and dynoed it at 140 something hp at the rear wheels. I think even after a buildup the 289 didn't make much more than 200 hp at the rear wheels.