IMHO, that is definately not JB nor brass. Appearance is like stainless so it is nickel rod. You need to be careful.with cast iron as it can crack at the edge of the weld as it cools. You must take steps to stress relieve the area. I could see preheating the area and using a tig with a nickel rod to fill the low spots. Best to have an experienced welder do the job, and then take the block to a machine shop to have the deck surfaces milled to the same height and probably bore and hone to size. Might be able to get away with diegrind the high spots of the weld and a light hone.
OP says he found a 5.9 Magnum so that is a good way to go. Welding and machining will be more than the good Magnum.