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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

A sure test to see if you have a manifold to head port vacuum leak on the crankcase side is to block off all the openings on the valve covers ( breather, PCV, Oil fill cap, Etc. ) and see if the idle improves with everything blocked off. If it does, you have a vacuum leak to the crankcass.



With everything blocked install a vacuum gauge on the dipstick tube and see if you have vacuum with it running from the valley side of the intake.
Also use a water bottle and spray on the header tubes after it runs for a little bit and make sure each tube is getting hot, great way to find a cylinder down.