BIG! There is not a good way to make a weld with steel to CI the way you plan on doing it. Rebar is mostly very high carbon steel recycled material. It can also be mixed carbon content. A Nickle rod is commonly used, but you need to butter each piece without overheating the CI or totally preheating the entire CI part to butter up the attachment points. The welds you make are then nickle to nickle. Personally, I'd make a frame with a ring where you could set the brake drum. IMHO, I would not waste time trying to weld to the drum. That way when you burn it out, just drop another one in the ring!
Now, here is the clincher! That drum may be CAST STEEL!
If this is the case, 7018 should be the rod with preheats on both the rebar and the drum. Grind the drum and look at the sparks. CI sparks will be orange/red streaks with no starbursts. Steel sparks will be more yellow with starbursts. The higher the carbon content of the steel, the more starbursts.
Do you have access to coal? Anthracite is what you want!
Keep asking the questions, BIG! we have more than 40,000 experiences to draw from!
GregH