Will 400 Cc's-425 Cc's Give Me a D Cup? If So, Are They Too Big for my Frame?

You are wise to pay attention to the rib cages and shoulders as they both are factors in determining the size of an implant.  Do not allow yourself to have size determined solely on cup size.  Bra manufacturers vary greatly in their sizing.  A “B” in brand X is a “C” in brand Y, etc.  The sizing is all over the chart so decide the size on how the fit is relative to your frame.  Your Board Certified Plastic Surgeon will be able to help you determine how big you have to go to fill out the loose skin.

t is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.