Rough calculations indicate your engine is making 680 horse power. Its does not take rocket science to make that kind of power with a big inch BBC. Thats only 1.35 hp per cubic inch.
Are you sure you don't have some type of A/F problem when you are putting the engine on a load.
And yes you are correct, it is pretty easy to build a small block that makes 680 hp. Seems to me that you should be making 750 to 800 HP. Have you pulled a plug after a run to inspect its condition? Is it running really rich?
Its gonna take 800 horse to break into the nines in the quarter mile, but you'll still be running about 6:30 to 6:50's in the eight.
If you can't drop any weight it will take a 1000 HP to put you in the high 5:90's with a 3400 lb car
Throw a 100 or 150 shot to it and see what happends. Maybe your fuel just isn't atomizing correctly.