I guess you tired almost everything that I would have done to try to pick the engine up power wise....and it didnt work.
But Like mentioned, that engine might perform as well as any other engine that makes 600hp on someone Else's dyno.
Those heads on that large of a engine will make more torque, than all out hp...
You need a way to extend the RPM range another 700-1000 rpm....then hp has no other choice but to follow.
I remember the first time down the track with a 4.15 stroke engine, and box stock edelbrock heads....It pulled good till about 5800 rpm's..Then it was literally like running the car against a brick wall...the nose fell down...I knew it was going to be a mid range rpm engine...