What carb are you using? Also how accurate is the dyno? I've seen some dynos put out "happy" numbers but when the cars went to the track the ET is not near what it should be. Maybe this dyno is "low"?

For comparison my engine is:

440 stock stroke
10.9:1
TF 240 heads
MP .557 cam
Crane Gold 1.5 rockers
Victor 340 intake
Proform 850 carb
Hooker 2" headers

Everything was out of the box and bolted on, no port work, gasket match, etc. Engine goes 6.70s in the 1/8th in a 3150lb car. According to Wallace Racing Calculator it's making 504 Flywheel HP.