You do not want to go lower on the thermostat, this will create unnecessary problems if the engine runs too cold consistently. Citric acid flush is very effective, I have never tried the prestone flush, and I probably wouldn't want to use it. Drain plugs are on the block (Sorry I do not know where on the block on the 12 cylinder) and at the bottom of the radiator.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record