Real Iron ? Where in the world did you find real Iron? Or are you working with mid steel? Either way, the thickness doesn't much matter as the skelp cand be thinned or thickened at the forge, in fact unless you like to grind or file a lot, thinning some parts and "gathering" other parts is part of the forging process. To answer your question, if you are forging, I'd start with 1/8" material.
John