Yup. Like Dodgestang and Starfury said, fudged power ratings. Not only were they rated without accessories of any kind, but they were often rated at gross horsepower and torque, which also excludedthe power lost to internal friction from bearings, rings and pistons. Since the friction rating was a purely theoretical calculation they could fudge it quite a bit. The gross ratings were supposedly the theoretical power ratings in the combustion chamber, excluding inefficiency from rod/crank angles. That's why engines like the Chrysler 426 Hemi were rated at 425hp one year, and then the EXACT same engine was rated at 375 the next. 50hp alone was lost just to friction and accessories. And even then those ratings weren't that controlled. The new(within the last few years) SAE net requires a VERY rigorous and controlled testing environment(it 100% eliminates the manufacturer's ability to fudge the power rating in any way), that's why a modern net rated car with 300hp(the 05+ Mustang GT's for instance) with 3,500lbs are considerably faster that older 400hp rated cars with 3,000lbs. It would depend on the manufacturer, the year and the vehicle. Often they'd under rate the performance cars to keep insurance down, and over rate the regular cars to sell them more easily.

That said, a stock K code with 271hp, was more likely about 220-240 true net hp, an A code 225hp was realistically more like 180-200 on a good day, and a C rated 200 was around 150-180. These are approximations though, since there are very few truly original Mustangs left with few enough miles to test them to know for certain.

Look at what BA said too, a 4bbl 289 was dyno'd by an owner at 130rwhp. When you consider heavy oiled inefficient transmissions of the day, and account for approximate power loss, that's only around 160-170hp at the crank.

Realistically a stock 289 with intake and carb is looking at maybe 180-200 at the crank, and like 150 at the wheels(on a good day)....so basically similar power as a lot of modern 4 cylinder "performance" engines.

The sad but true truth of our old V8 mustangs. Keep it on the DL dont want the tuner crowd to find out lol:p

Yeah, but the stock 351W Mustangs were some of the fastest, I think even back then they managed to get a few to run high 13's...on the crap tires of the day. But most 289's were pretty weak.