Top of WP goes to top of radiator.
As long as you dont have a closed heater control valve, coolant will flow through that circuit to fill the motor. But when filling, I left the radiator cap off until the thermostat opened and kept it full.
Note that with an LS motor, the radiator cam will not be the highest point in the system to bleed air out. I had to add an inline radiator cap housing over the top of the radiator. The external tank connected to the heater hose works too.