I searched the archives too. Don't own a 420 but have been considering one, hence the curiosity.
In one post, Benzmac indicated the block as well as the radiator could be drained. When I change my coolant, I remove the T-stat so I can forward/reverse flush.
The 103 motors have a 19mm plug on the passenger side that's easy to get at. Your V-8s engine plug may be a bit harder to get at? If you don't receive anything here, you may need to refer to the 126 CD-Rom.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.