[71GMC_3/4T]

Lightweight heads on a SBC are prone to cracks. These are usually through the exhaust port into the water jacket in the center two cylinders.
Take them off and fix them before you end up filling a cylinder with water, hydraulic locking it, bending the rod, breaking the piston, etc...
A good shop can magnaflux them and show you the cracks. If they aren't too cracked, some shops will redo the valve seat, which sometimes will hold up.