Welding on cast iron requires the part to be preheated. If the part isn’t preheated (and even if it is but doesn’t get hot enough) the application of the electrode will generate heat and when that hits a cold spot will crack the cast iron, further compounding the issue.
If you opt to go ahead with the welding, we recommend our 55% Nickel “Super Ni-Cast” rod, or our Super NI-Cast 99% Nickel. The 55% has a higher tensile strength and typical applications are Grade 30, 40, and 50 cast iron, and the 99% indicates a typical application would be engine blocks. See our entire line of cast iron electrodes here: https://www.forneyind.com/products/welding/stick/stick-electrodes?L4=cast+iron+electrodes.
Another option would be to drill the holes out, tap them, and put a threaded insert in. The stud could then be threaded into that and would eliminate the need for the welding. Your local parts store should be able to assist with that if that’s the way you decide to go.