Looking good! One thing about Iron Man armor, is that it is very close-fitting. Essentially, every part needs to be snug around your arms, legs, chest, waist, and head. I think the bicep is too big, but the shoulder bell is perfect. Though it is hard to tell without the rest of it. The edges around the top and bottom of the bicep are just there for support, so if they make the part too tight, just cut them to fit better.
P.s. My helmet is free now, so if you want a smoother build, get the file here.