How do I route the LS steam vent tube when the radiator has no steam port?

Tie it into the upper radiator hose with an adapter that has a steam barb. Speedway's LS swap radiator hose adapter (listed at $24.99 in its article) fixes both problems at once: a 1/4 in hose barb takes the steam line, and it adapts the LS water pump's 1-1/4 in upper outlet to the 1-1/2 in inlet on a typical radiator.

The steam line runs from the heads to the barb on the adapter, so trapped air vents into the upper hose and on to the radiator. Speedway notes this adapter also opens up far more radiator choices, including its budget universal radiators, because it matches the common 1-1/2 in inlet.

Why you need it at all: On most LS engines the thermostat sits low, on the water pump, instead of up top like older small- and big-blocks. GM added steam lines in the cylinder heads to vent trapped air and keep hot spots from forming at the top of the engine.