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

It is difficult from the photos posted to see the concavity of the central chest. It does not look bad in the photos. As for implants, those are very large and they will not look natural.  An exam in person would be best to assess a good volume. You can also do a baggy test to get a rough idea.

With your relatively small amount of natural breast tissue, I would strongly recommend a silicone gel implant to reduce the chances of visible rippling. Given your dimensions, I would go with 410-440cc moderate profile gel implants in a submuscular pocket through an inframammary incision. This will give you a very nice, natural appearing result, although it is not possible to promise an exact bra size since these vary a great deal.