Vacume leak on the intake, possible miss adjusted valves, or both!
The intake needs o be checked put it on dry checkit with feeler gage top bottom side to side, do not use cork end gaskets if it is aluminum just use gasket sealer . nice 1/4 bead will work great.
If intake checks out, use small amount o sealer around the intake ports top bottom of gasket .
Most small blocks love to leak on the valley side