While I probably am not the best person to offer advise on heat treating, IMHO, Ravenshear's instructions leave a lot to be desired. He suggests heating the spring to nonmagnetic, quenching in 30 wt oil, and tempering in burning oil.
The problem with heating to nonmagnetic is that nonmag is only about 1425 degrees, and not hot enough to produce the desired results. IMHO, steel needs to be heated a color or two above nonmag, to 1550 F, then soaked at that temp for the appropriate time.
I suggest quenching 6150 transmission oil, as Wick mentioned, then tempering in a temp controlled lead bath.
That said, find a piece of sheet lead to make a pattern of the cast spring before it breaks. Bend the pattern flat and trace it onto sheet steel for a permanent template. Mark the bend on the template and you're good to go.
As an addendum. While I have had limited success with tempering springs by heating them on a steel plate, I do know people who have used this method with good success...with 1070 series steel.
IMHO, a bright blue color is only about 570 degrees, F. Not nearly enough heat to properly temper any spring intended for use on a ML. One must have the ability to discern the subtle gray colors as the temp passes from bright blue to the light blue and through the light gray spectrum to an increasing slightly darker gray, which is about 750-800 degrees F. Otherwise, a new may spring in in order.
http://anvilfire.com/index.php?bodyName=/FAQs/temper_colors_hardness.htm&titleName=Temper%20Colors%20and%20Steel%20Hardness%20:%20anvilfire.comNotice that this chart includes Rc hardness for the various the temp colors, for those plain carbon steels.
One last word of caution in using tempering colors, they must be viewed in consistent lighting to produce consistent results. IMHO, the colors will appear differently in different lighting conditions, which can adversely affect the final result of your heat treating.
God bless