Well, not to knock down HP numbers but they all have to be taken with a grain of salt.
The type of dyno wasn't mentioned, the brand, the method of testing, the correction factor etc....
All these things have an affect on the numbers.
Some things I can tell you, that number if indeed correct, was likely done in an engine dyno cell before installed in the car. Again not knowing the brand (DTS is one I'm familiar with and they are stingy) and not knowing the setup, some dyno's won't allow testing with stock exhaust manifolds. They lack the room, so sometimes specifically made dyno headers are used, which actually make more power than chassis headers do. That's just one scenario.
Another could be that the engine was run with no accessories, no mechanical water pump, no mechanical fuel pump etc...which is common to ease hook up and speed up the dyno process. In which case you could see a 20-40 HP bump.
Without knowing any of these things it's too difficult to judge.
Dyno's are tuning tools more than anything, the number is just a guideline. On any different dyno or operator the numbers could be different, as much as 25HP.
To make that kind of power the heads would only need to flow about 260 cfm with alot of other factors that play a roll.
With the rules that allows a 1.5 bump in advertised compression and this car being purposely built to run the class, you can bet you're looking at a 12.25:1 engine, and probably a max bore of .070 would also help to increase the head flow by unshrouding the valves. From there it's attention to detail. Things like zero decking the block, CC the heads and make sure all chambers are identical, making sure all rocker arms are the advertised ratio, checking cam lobes, degreeing the camshaft, the small 1/16" ring package helps, and just lots of other little things that a good engine builder will do, even on a stock type rebuild using stock parts to extract the most out of what you have.
Keep in mind none of these cars actually had the compression ratio advertised from the factory, and that goes for any OEM engine back then. Most fell .25 to .50 a point below advertised, since the pistons were generally .020 to .025 down in the hole, and cylinder heads CC all over the place, you have core shift issues with cast parts, intake manifolds that don't match up very well (pontiac is notorious for that), deck heights that vary from front to back from core shift which also affects compression ratios from one end of the engine to the other. Etc....
All these things are corrected (or should be) even during your most basic rebuilds, so in essence when finished it should run better than it ever did from the factory because you are correcting mass produced issues that weren't a concern back then. A well done stock rebuild will boost the HP without any real tricks.
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