Wallace says 6.73 @ 101 with a 3300 lb car is 535 hp. So I agree that you are 75 to 100 hp less than I would expect from that engine. I can't tell you where the missing hp is but you should be able to figure it out with common tuning tools. Run the lash, pull the plugs, do a compression test, check the timing, etc. If the engine is in perfect shape then you have a big mismatch with your parts. That cam is too small for a race engine but it should still be making closer to 600 hp than 500 so I'm guessing that you have a mechanical problem of some sort. Or it could be bad gas. I've seen engines lose 100 hp on the dyno with old gas. The engines run fine, they just don't make good power. You can tell by the exhaust note if you listen close but other than that they seem to run fine. It freaked me out the first time it happened but after running into it a few times we started to get better at figuring it out. So get a new pail of fuel and try it again if you don't find anything else wrong in your inspection.