I've had this sculpt pending for a bit and will get started this weekend, and plan to have the first pull by late July.
The problem with having a wearable arc reactor is that often they are thick, or are too obviously an external object and not part of the chest. To fix that, I'm planning on creating a reusable prosthetic out of silicone to fit the standard size arc reactor (with a 3.5" diameter, as in ThrowingChicken's excellent kit).
In the movies, Robert Downey Jr's chest was actually latex that was layered up each time and then airbrushed:
I'm intending to make it a smaller piece that will fit between the two breasts (not the entire chest), by doing a two-piece mold, intended to fit the arc reactor kit.
Photos to come this weekend.