Biggest tire and 15" wheel possible on a 1966 Dart?

I think they will rub.
I raced a Dart in SCCA, and we ran 15 inch, but had to replace the whole well, and bow the body to fit. This was during the late '60s, and early '70s, so even racing tires weren't as wide as some street tires now. Granted we ran a decambered spring, and the car sat lower, but with the softer stock springs you'll have a lot more motion.