Probably a bent valve cover from overtightening
Replace or try to straighten then tighten to 10 inch lbs. only
if you have a leaking water pump gasket i thing bay a new one is better
your heater core is leaking
If you are sure that it did not freeze and crack the block, Then the intake manifold gasket is leaking. That is a common problem with the vortec Chevy engines. You need to replace the gasket with a factory Chevy gasket from the dealer.
You either need a head gasket or intake gasket.
The head gasket on a 1999 Chevy Malibu is replaced by removing the coolant, intake manifold, and retaining bolts. The cylinder head can then be removed, the gasket replaced, and everything reassembled.
NO NO, That would be a intake gasket leaking, not a head gasket. You also should check where the heater hose hooks up on the intake, it may be leaking.
bad head gasket
Sounds like head gasket is leaking
Heater return pipe or hose, leaking intake manafold gasket.
possible valve cover leaking. The header would burn off the oil dripping right away and make you think it is leaking below the header But if it is leaking below the header as you stated it is probably coming from the manual fuel pump gasket or the pump itself or the cover plate if its been removed and replaced with electric.
Check for a sickly sweet smell. The heater core is probably leaking coolant into the floor. The core is not repairable but can be replaced.
Intake manifold gasket