6ft, 135lbs, A to C: 500cc too large?

While at 6'0" your frame could certainly tolerate a 500cc implant I'm not sure you base width will. The base on a Mentor HP 500cc is 13.2cm. The 600cc is 14cm. You can use these but you are going to have a lot of implant displaced laterally as there just isn't that much room between your nipple and your sternum where the muscle inserts. If you want that much volume you might want to consider the ultra high profile implant. There you can get a 595cc implant that has a base of 12.5 cm

Breast Implants: Best Size and ShapeThese are very large implants. First of all, what is your bra strap size? At 12 cm diameter, you can’t be more than a 34 possible 32. Average volume is 325-350 for a 34 a to c and 250-300 for a 32 a to 32 c.Please also consider anatomic gel implants by Sientra. You may also need a minimally larger implant on the right looking at these photos.

Thank you for the question and pictures. Unfortunately, online consultants will not be able to provide you with specific enough advice to be truly helpful. Given your questions and concerns, you'll be best off spending additional time with your plastic surgeon communicating your goals carefully. Keep in mind, that there is no reliable correlation between the breast implant size/profile use and specific cup size achieved.
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're 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. I have found that the use of words such as  “natural”  or  "too large" or "C or D 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.
 The use of computer imaging technology may also be very helpful during this communication process.

3.  Once you feel you have communicated your goals clearly,   allowyour 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.
I hope this (and the attached link) help.