What grade of ASTM A572 are you welding?
AISI 1045 merchant grade steel has a carbon content of about 0.45%. That is on the high side of what I would like to see for the carbon. I like to see it 0.3% or less.
The filler metal will be dictated by the grade of ASTM A572 you are using. You can undermatch the filler metal to match the lower strength A572 as long as you use a low hydrogen technique (fresh EXX18 or bare filler metal) and high preheat.
Best regards - Al