I put a copper radiator for a ~1972 Dart (square top) in my 1965 Dart. It was the first one Autozone listed w/ a passenger side lower port, and I wasn't resourceful enough for ebay then. The lower port is 1.5"D, so probably intended for a slant. I actually used it for my 273 V-8, but w/ the later alum water pump, and had to adapt to 1.75"D. I recall having to drill 2 holes to match the car's bolt holes. The lower port doesn't angle like most slant radiator do, but should still work. For lower hose, lookup the PN I posted for my 64 slant.
But don't feel so limited. Many radiators could work. My 64 slant has a thin radiator from who knows what. It came with a Honda battery and M-B pusher fan (and no pulley fan). Most all cars I have seen have a 1.5"D upper hose. Think where you want the upper port for any later accessories you might add like AC or power steering. A center port works nice for a slant, since you just need a little 90 deg hose and those will never become "not avail".
Before all that, try steps 1 & 2. Verify your temperatures with an IR gun (cheap at HF). Remove the radiator you have and fill w/ citric acid solution (ebay or grocery store) for a few days (or Prestone flush), then flush. Verify it flows well using a garden hose, and also back flush it. You may find pinhole leaks that were ready to fail, but that is a good thing. Also, remove and test the thermostat.
