I Would Like Cleavage but Have a Wide Sternum - Will I Get Cleavage with 300cc Silicone Overs or is 350cc Better?

Post-operative cleavage will depend on your pre-surgery cleavage. You have wide-set breasts, which means you'll probably continue to have wide-set breasts after surgery. However, while you may have a bit of improvement, it may not be as much as you want. Unfortunately, your surgeon can only do so much to give you cleavage before it looks abnormal, since the implants have to remain centred underneath your nipples.  

It 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.

Breast augmentation does not necessarily give you cleavage. Some people have breasts which are naturally closer together and will have more cleavage after breast implants are placed.  Based on your photos I feel it will be difficult for you to have cleavage after breast augmentation, although your breasts will be closer together.  You appear to be thin with minimal breast tissue so I would also recommend silicone implants to reduce the chance of rippling (compared to saline implants).

Cleavage is best obtained with the use of a good bra.

Best wishes,

Dr. Bruno