Radiator in 63 Dart GT

Surely that radiator was rebuilt as the cores eventually clog or corrode thru. Those round-top radiators tended to split at the seam. For me, in the middle on the fwd side against the radiator support where you could barely see the coolant dribbling out at first, at least 2 times that I recall in my 1960's Mopars. A shop which rebuilt one of mine said they soldered in an internal support to prevent that recurring. The 1970's square-top copper radiators seemed to not have that problem, but are far from "correct" for your car. Many people go with an after-market aluminum radiator. For a non-AC slant car, I'm pretty sure an OE radiator would be less wide ~19". Later non-AC slant cars had a filler plate on the L side to fill the 22" wide opening. I think 1963 cars had a 19" wide opening in the radiator support since there was no V-8 option until 1964.