Hey weaver,
Several years ago reading up on cast iron, I found interesting data & results with a technique called: "Cold Welding Cast Iron With MIG". There were references in an older Lincoln book & some postings on several websites. I went to my brothers' engine shop & picked up a few intake & exhaust manifolds he had scrapped & did a couple months of experimenting. I made notations in my logbook w/results & parameters used.
My technique involves a good cleaning of the weld area w/wire wheel or light grinding, a good bevel of the entire crack, & either backgas or solar flux. The process dictates tacking in several positions to secure the joint & then short beads, not exceeding 1/2" to complete the joint. Each application of weld has to be peened immediately after welding & allowed to cool back to room temperature. This keeps the casting cool & after each bead, you can remove your glove & lay your hand on the cast & the cast would only feel warm to the touch. This technique eliminates the dreaded "tinkle"(another crack) that occurs with heating/cooling of cast iron.
I use 309L(.01 carbon) SS wire, .030, 98/2(AR/CO2) @ 18cfh, heat between 80-100A depending on thickness, & "pull"(drag) the puddle. You can use any combo of gas not exceeding 5% CO2(ex:95/5). So far, I have had complete success & no issues. Attached photos are a couple exhaust manifolds I did for a foreign car dealer from (2) Rolls Royce autos. The dealer was quite pleased as replacement manifolds were very expensive.
Hope this may give you some food-for-thought....
Denny