RB,
Why not obtain the mill test reports (MTR's), determine CE using one of the CE formulas, determine how much preheat per inch or centimeter of material thickness? (Take into account the second paragraph of my post). Works for me everytime, and gets approved always (so far...).
Selection of the electrode should be based on criteria other than the base metal composition. Will (is) your weldment be subject to specific design criteria such as mechanical properties (toughness, hardness, ductility, wear, abrasion, etc.) cyclic loading (stress reversal, corrosion, or other service-related requirements), etc.? Undermatching tensile strengths may be permissible. Too many variables without more info on your end.
Similar/recent post about these subjects by JW. Some valid opinions by others.