LS steam vent: do I need it?

Yes. The LS thermostat sits low on the water pump, so air gets trapped in the heads. Trapped air causes hot spots and overheating. The factory steam line vents that air back into the flowing coolant. In a swap, route the steam line to the steam port on an LS-swap radiator, or use a hose adapter with a steam port if you're keeping a universal or factory radiator.