RE: 3.5-Vacuum Leak
Don't put sealant on there. Inspect the sealing surfaces carefully for nicks and gouges. Inspect the gaskets for integrity. Replace any that are doubtful. Inspect all the hoses for cracks and hardening.
Inspect the check valve at the brake booster for cracks or leaks. Put a finger over each hose connecrion to check for leaks while the engine is running.
Check the EGR and PCV circuits.
You may be able to find or confirm a vacuum leak by listening through a tube. Put one end of the tube close to your ear and check suspected spots with the other end of the tube.
Spray carb cleaner on a suspected area. If the leak momentarily stops, it's confirmed.
Your upper manifold may be warped and not mating properly with the lower manifold. If you can get hold of a precision straightedge, check the flatness of the mating surfaces.
There's a proper sequence for removing and tightening the bolts. Make sure you follow it properly.
Engineer and current owner of two Sebrings.