What cup size would 350cc saline, under the muscle implants put me at?

Thank you for the question.   There is no accurate/reliable/predictable correlation between size/profile breast implant utilized and cup size achieved. For example, I do not think that the generalizations of "200 cc equates to 1 cup" hold any validity or are helpful in any specific or practical way.  In my practice, I ask patients not to communicate their goals, or evaluate the outcome of the procedure performed, based on cup size references.
Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, in bra sizers, and computer imaging)  as well as careful measurements (dimensional planning) will be critical.
 ***Given that your surgery is coming up soon, I would suggest that you spend additional time communicating your goals/concerns directly with your plastic surgeon, preferably prior to the date of surgery.  It will be important for many reasons, that you feel comfortable that you and your plastic surgeon are on the "same page" prior to proceeding.
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals. 
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. For example, 
I have found that the use of words such as “natural” or "D or DD cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this (and the attached link, dedicated to breast augmentation surgery concerns) helps. Best wishes. 

Since bra manufacturers differ greatly in their sizing, it is impossible to predict bra size after breast implant surgery.  We try to emphasize choosing implants to give the best "look", regardless of the bra size.  It is much more important to have a look that our patients enjoy then to try to conform to some random bra size from a bra manufacturer.Typically, 200 cc in breast implant volume will increase bra size one cup, so 350 cc should increase the bra size about 2 cups.  If you are currently an A cup and wish to go to a D or DD, then 350 cc will not be enough.  I would suggest another visit to your plastic surgeon to discuss your concerns.

Thank you for your question.  Cup size is very difficult to predict with accuracy because different stores label the same bra different cup sizes.  The issue is how large of an implant  will your breast and body accommodate.  I assume that you're surging basis 350 cc based on a measurement of the base width of your breast.  If you want to go larger than that then a high profile implant will likely be required.   Please discuss her goals in detail with your surgeon.