Hi Chris,
I've got AP armor, too. I read your post wrong at first and was posting pics of my bicep pieces, but then realized you have the opposite problem. I've got scrawny biceps. You don't need to see those pics.
As far as I know, you can shim them. The cover strips will help you, too. I think the usual cover strip for the arm pieces are 15mm, but I think going up to a 20mm cover strip is ok, but get some morre opinions just to make sure. I believe if you can get the shims to line up flush to the original biceps pieces, you should be ok. And, then use slightly larger cover strips to help hide the shims.... Good luck! Keep us updated when you get the time.