The armor does look a little big and could help with some reshaping with a heat gun to curve better with your chest.
Ugh. I was dreading having to do the back plate since I'm hand shaping it and that is one of the largest pieces.Yeah, a good place to start is get the collar and backplate figured out and then add the chest plates. The collar defines where the chest plates sit and the backplate can change how the collar sits. So it sometimes works better to do these two pieces first and then move to the chest and belly plates. Not as fancy and showy as the chest but the end results can be much better.
Having a buck to form over really helps.
I have a duct tape mannequin that I've been using as a reference. Unfortunately it doesn't react well to hot plastic. I wonder if there is some material to heat-protect it or some easy way to convert it to a usable buck.