The key being "minus the Nascar mandated restricter plate." I have a lot easier time believing those numbers on a non-rules engine, meaning the restrictor plate isn't in place when they do those dyno pulls. The engine probably isn't running the power-steering pump, alternator, or water pump either (for the record, neither were Barton's PST engines back in the day).
That's what burns me about dyno-cell numbers... what were the conditions? You watch a dyno pull on Horsepower TV or Musclecar, and yeah, they made 520HP--with no alternator running the ignition, the ignition system itself being a $3000 system that can generate lightning that would impress God himself, no power-steering pump, etc. They do usually run the water pump from the crank, but that's it and it doesn't even have a fan on it though it probably will in the car. If I were going to dyno my engine I'd want all accessories in place and running their attendant systems. Of course, that's a bunch more money for a fella to dole out in dyno setup time too.
Now, chassis-dyno numbers? Those I'll eat right up!
I understand why NASCAR ended up the way they did to some extent. The car market deviated from their stock in trade, pardon the pun, but they still should have kept up with the times to some extent. Damned-near everything was fuel injected by 1991, and for the past five or six years the used-to-be-big 3 have had fullsize RWD offerings that fit the standards of yore. They have production engines that could be made to compete (LSx, Hemi, and Mod). So what if they're four-doors? I'd rather see production bodies run rather than cars that don't exist like RWD Monte Carlos or two-door Chargers. If a production body isn't slippery enough to compete, then the company needs to dream up some goofy concoction like the winged B-bodies or the Torino Talladega or King Cobra (I know, the latter never competed and was just an exercise) or the Pontiac Grand Prix 2+2 and produce enough of it to be legal. It'd be a lot more interesting than this cookie-cutter stuff they run now, and when NASCAR was at the height of its popularity a copy of the car that ran on Sunday may have damned-well sold on Monday. Why not change the series over to the ponycars, which are far-more relevant to performance fans these days?
For the record, I share much of this frustration with NHRA Pro Stock. Please direct me to the dealer that can sell me a 500-inch RWD Stratus or Cavalier, please. At least the doors and roof have to be stock and the doors even have to work, but still... they should change the name to Pro Stock Doors or Pro Carbureted Gasoline. If they're going to keep that name, that class should be Camaros, Mustangs, and Challengers with production-based unibodies and engines, a stated cubic-inch limit, MPFI only regardless of configuration (dual-throttle-body sheetmetal tunnel rams would be fine by me).