The cast iron rods I use are nickel based and suitable for AC and DC, never heard of any other suitable alloy.
Welding is easy enough, but it is important to pre-heat the barrels (250-250°C, kitchen oven at max. temp.) and only do stitch-welding and no continuous seam, as this would bring in too much heat into the fin, thus leading to a high temperature gradient and finally causing cracks, doing more harm than good.
It is advisable, if possible, to carefully knock on each stitch with a small hammer carefully immediately after welding to release internal stresses.
Let the barrels cool down very slowly after welding.
This way I welded two fins back on the barrels of my A10 with just a welding spot about every half inch or so. Grinding V-grooves in first makes for a good joint and less dressing work.
Brazing definitely is an alternative, but as applying much more heat is involved, I think the danger of distortion is higher, especially when working sclose to the bore. Doesn't matter if you do a rebore afterwards though.
Cheers, Markus
Ken was faster