To completely drain it, you need to pull the knock sensors. This is not easy to do. But to get a good coolant mix, you can do less. There is a procedure for coolant fill for the L36 in Techinfo. http://www.bonnevilleclub.com/forum/...=article&k=100
That procedure helps to eliminate trapped air in the cooling system. Please read that first.
There is probably an easier way to do this, but here is one way to fill your engine to achieve a 50/50 mix. First, remove the thermostat and drill a 1/16-inch hole in the thermostat flange. Drain most of the engine and all of the radiator by disconnecting the lower radiator hose. Disconnect the heater hoses and blow into one of them to displace the coolant out the other hose. That will get rid of all the water you can without pulling the knock sensors. Maybe two or three quarts of water will remain in the block.
Techinfo says the cooling system capacity is 13 quarts. Assuming you have pure water in there now, you need to add 6.5 quarts of pure coolant. I'm going to guess and say the capacity of the radiator is about 4 quarts, the heater core and hoses 1.5 quarts, the recovery tank 2 quarts, and the remainder, 5.5 quarts, in the engine. The exact values don't matter so much as long as you get 6.5 quarts of pure coolant in the system.
Start with two gallon jugs of pure coolant and one empty gallon jug. Pour half a gallon of coolant into the gallon jug and add a half gallon of water. You will now have 6 quarts of pure coolant and 4 quarts of 50/50 mix.
First, with both heater hoses disconnected from the tensioner assembly, fill the heater core and hoses with pure coolant. When the heater core and hoses are full, attach them to the tensioner assembly and secure the clamps.
Next, add pure coolant to the block through the thermostat opening. There will be a good bit of pure water in the block that does not drain when the lower hose is disconnected. Let'* assume that will mix with about the same amount of coolant in the block. Keep adding coolant until the level reaches the lip of the thermostat recess.
Next, add pure coolant to the radiator watching the thermostat opening carefully so that it does not overflow. If the radiator is not full by the time you have added 5.5 quarts of pure coolant, add pure water to the radiator until it is full. You should see a lot of air bubbles come up at the thermostat opening as you add fluid to the radiator. When the coolant begins to rise out of the thermostat opening, install the drilled thermostat and the top radiator hose. Then keep filling the radiator slowly. Pump the top radiator hose with your hand to expel air from the hose. Wait a few minutes, then add some more until the level reaches the hole for the recovery tank hose.
Fill the coolant recovery tank with two quarts of 50/50 mix or enough to fill at least two inches above the full hot line.
After bleeding according to the directions in the Techinfo article, and after a few days of mixing in the system, open the radiator cold and test the coolant concentration.