Want to flush coolant on SDL

[WANT '71 280SEL]

1. Where is the block drain. Do I even need to drain the block or can I just pull a plug on the radiator to empty everything out?
2. Does the radiator have some sort of drain plug?
3. I want to use a citric flush agent from one of the chain stores. (Prestone)
4. After I've some how drained all the coolant out, I add the flushing agent (through the upper hose I figured) and just water or a coolant/water mix?

I would then run the car with the agent + water(and coolant?)mix with the heater on high? Don't I want to get the car up to temp? Should the thermostat be removed for this? Any special parts needed to temporarily remove thermostat (gaskts, o-rings)

I appreciate any help.
Thanks
David