[TR] cleaning cooling system

[Tom Note]

Hello folks:
I have the radiator out of our '76 TR6 for installation of an electric 
cooling fan, and I'd like to flush the block based on the rusty stuff that 
came out of the radiator.
There's a large hex-head bolt or plug on the starter side of the block 
toward the rear. Is that a block drain plug?
If so, can the block be flushed adequetly w/o removing that plug? (looks 
like it's a bear to get at).
Anyone have a suggested method or sequence for flushing the system?
TIA,
Tom