This is about a 1971 LS5 (454 ci) cooling system.
The owner manual says the capacity is 23 qts.
I have flushed the system several times, through the lowest point I could find: The fitting just below the lower connection point of the radiator (bottom right).
The most I get out of the system, and therefore the most I could get back into it was approximatley 3 gallons (12 qts.
Any idea about this ? Any experience with a similar engine ?

remove the drain plugs on each side of the block about halfway back on the lower edge near the oil pan. Remove the bottom radiator hose if there is no radiator drain plug. flush well.

you wont get all the water out with the engine level

I lost a bunch of water when I disconnected the everything for the frame off.. then more when i moved it 7 miles, more when I moved the frame back, more when i disconnected the transmission and even more when i had it on the engine stand and turned it over...

it does hold quite a bit of water.. almost 6 gallons sounds about right.

Something else you can do to get the water out... use an air compressor... pick a point of your engine to release the water and then another place to add the pressure

Don't forget the capacity of the header core and heater hoses. I dropped a buch out of these long hoses when removing my engine.