They are very reliable and are available for the 65/66.Plus you can use the deep cover from your old one for that original look.If you need a new regulater,its the only way to go.
I was actually asking for research info - more of the kind like Fred's answer. In other words, could you order a 65/66 Mustang with a transistorized regulator from the factory. Sounds like the answer is "no".