Prabhu,
SA-537 Cl. 1 & 2 both have P.No. 1 with Group No. 2 & 3 respectively.
Additionally, there is no difference in chemical composition however minimum tensile strength of Class 2 material is 70 ksi versus 65 ksi for Class 1.
Based upon this, if the material has to face low temperature service requiring higher notch toughness then both classes can be welded using E8018-C3.
If higher notch toughness is not required, use E7018 for Class 1 and E8018-C3 for Class 3.
(attached file contains a comparison of two materials' welding properties)
Octane