At the front of the Ford Racing catalog there's a section explaining the crate engine process and dyno testing. They do not specifically state whether the crate engine hp numbers are gross or net.
Here's some additional info:
In 1972, American manufacturers phased in SAE net horsepower.
This is the standard on which current American ratings are based.
This rating is measured at the flywheel, on an engine dyno, but
the engine is tested with all accessories installed, including a
full exhaust system, all pumps, the alternator, the starter, and
emissions controls. Both SAE net and SAE gross horsepower test
procedures are documented in Society of Automotive Engineers
standard J1349.
SAE Gross Horsepower
This is the old process that American manufacturers used as a
guide for rating their cars. It was in place until 1971. SAE gross
also measures horsepower at the flywheel, but with no accessories
to bog it down. This is the bare engine with nothing but the
absolute essentials attached to it; little more than a carb,
fuel pump, oil pump, and water pump. Because the test equipment
on the engine is not the same as in SAE net, it is impossible to
provide a mathematical calculation between SAE net and SAE gross.
As a general rule, however, SAE net tends to be approximately 80%
of the value of SAE gross. SAE J245 and J1995 define this measurement.
Without knowing for sure, I would assume that crate engines are rated at gross hp. There is no way for the builder to know what kind of vehicle the engine will go into and even if they did, it would be totally impractical to emulate even just the exhaust of whatever car the customer said they have.
The magazines do gross hp as well. Remember that article about a 400hp stock cammed 5.0 in I think MM&FF? Sure was easy to get 280hp with a mild carb, intake, and headers on a 225hp stock 5.0 engine, right? Well, all it was driving was a water pump and had no intake snorkel or exhaust.
There were also differences between the engine tune. I forgot the details, but for SAE gross measurements, the engine tune was basically according to the "blueprint", whereas the for SAE net measurements, the borders of the tolerances of the tune were used, like ignition timing at 5% from optimum or something like that. This was used to account for production and installation variations and wear. In other words, SAE gross specs were kinda like "300 hp on a good day", and SAE net are like "200 hp guaranteed, more in most cases".
This also explained that although cars got noticably slower after 1971, the difference was not as dramatic as the HP numbers would suggest. That is, for somewhat comparable engines, like a 351C 4V from say 1971 and 1973.
When it comes to accesories, the alternator is usually brought up, but an alternator doesn't use all that much power. If you take a typical (in those days) 65A alternator which gives 14V, the electrical power is 1,2 hp. Electrical machines typically have 70% efficiency, so including some losses in the belt drive, the loss may be 2-3 hp, MAX. A lot less when it doesn't charge.
Installing a stock exhaust system has of course a significant effect on power, especially those old ones. A mandrel bent 2.5" system on a modern Mustang is a lot more efficient.
Funny that the Ford catalog mentiones the starter. IMO, it just hangs there during a dyno test, doing nothing.