[Blas]

There should be a drain plug located on the lower connection of the radiator for your car series. Where the hose attaches to the radiator on the driver’s side under the car. Very early Superformance cars did not have a drain plug and you needed to remove the lower hose to drain the system.
A little penetrating oil overnight on the drain plug is a good idea too before opening it.
Drain and flush the system as necessary. When refilling, I add the proper amount of antifreeze first first based on the capacity of the cooling system, because there is always a little water trapped in the block that further dilutes a pre-mix. I use bottled distilled water when refilling and I also use the “Water Wetter” product too, your choice on that, and lot’s of opinions here. Some engines will have air trapped in their cooling system due to their design. Elevating the front of the car when filling the system helps to eliminate any trapped air. When refilling, fill the system and then run the engine with the cap off the expansion tank till the thermostat opens, the level will drop, then add additional fluid as necessary as the level goes down. A properly filled system will have fluid level up to the divider inside the expansion tank not to the top of the filler neck on the expansion tank. Remember to drain your overflow tank too if you have one installed. The car may spit water after coming up to temperature and pressure the first few times you drive it. It’s just equalizing the fluid level. If you see spikes in your temp gauge you have air trapped in the system. Warm a cold engine and carefully open the connection on top of the intake manifold that the water temp probe is in, air will escape due to the building pressure in the system and when the trapped air is expelled, antifreeze will start to seep out. Close the connection and you should be good to go. Clean up any spilled antifreeze as it stains when it dries. You know all about opening a hot cooling system I presume, and remember antifreeze is poisonous for animal’s and kids. Dispose of your old antifreeze properly too.