I would get a flushing tee connector that installs in the heater core line. Attach as specified, and start the garden hose flowing. Run the engine at idle to circulate the water.
You might remove the thermostat to allow flow through the radiator, but I think you want the flow to go against the output of the pump and flow backwards into the radiator, pushing stuff out of the heater core, block cooling jacket, and possibly the bottom of the radiator. The water will flow out the radiator cap port and go all over your street or drive.
Prestone made a kit with the tee, cap, hose clamps, and a diverter to stuff in the radiator. They also included instructions. It works to a degree.
If you remove the water(coolant) pump, you can flush around in the water jacket around the cylinders, and maybe rake out a pile of goop. You can remove the radiator, and flip it upside down and run water backwards through the tube, as much as will flow, to remove 'stuff' and sediment.
Then put it back together, add some chemical flush, TSP, oven cleane, vinegar, or your favorite cleaner, run the thing at idle until it has circulated as long as you want to play, drain, and flush to remove all the chemical used. Don't run it down the drain w/o neutralizing it. Baking soda for acids, vinegar or acid for bases. The store flushes will indicate proper disposition of their product, likely safe for waste handling via the street drain.
I would not run a higher concentration than specified. If you want, you can carry a small container of 100% in the cab, and add equal mix of water:coolant as needed.
tom
p.s. the pic was undecipherable to me, except showing a big blob of rust.
tom