Based on your pic that is the right hose which is the heater core outlet. Of the two ports you see at the firewall which go to the heater core left is outlet (return) and right is inlet (pressure).
The general instructions are for GM cars with the regulator radiator but you can improvise. It is up to you whether to disconnect the upper radiator hose or even remove the thermostat as some people do. BTW - Removing the thermostat on the 3400 engine is a PITA!
Removing the engine block drain plugs is OK if you can find and get to them but the instructions mention a way to remove most remaining coolant/water from the system by starting the engine and letting the water pump do the work. See step 5. Just to be safe make sure you run a hydrometer test of the coolant to make sure the premix is correct. 50/50 should protect down to -37 degrees F.
Good luck and let us know how it turns out.