I was referring to engines that you've received from other builders. Have you received those, then spent time to dyno them again before you put them in the car? You're still getting short blocks in and slapping the heads on, right?
I don't have my own engine dyno...I think it's a lot easier for people to believe you when it's a third party dyno. Needless to say, I've seen differences in dyno to dyno. Not saying anyone is inflating numbers, but saying that some read lower than others. I always dread posting my dyno numbers because they are generally lower than other builders' numbers. It's always weird to see your engine with selectively ported heads, a ported intake, larger cam, etc., and still make 25 hp less than other advertised engines.
That's fine because I'd rather have a 450hp engine that outruns the 600hp guy at the track.
I always dyno with the carb that goes with the engine. I don't always use the headers that go with the Cobra though, as on some dynos it's not feasible to plumb them up easily. Sometimes the customer doesn't furnish the accessories, so the engine can't be dyno'd with them.
You will also see big differences in chassis dynos. Even if itstock's loss was just 10%, IMO that engine will just not be over 600 lb-ft of torque at the flywheel. Even so, Cobras will usually see 15-20% losses due to the driveline and the restrictive sidepipes.
Basically, dynos are a necessary evil. I use them to make sure the engine is sound, no leaks, etc, and that's about where the line is drawn.