My 65 Dart w/ 273 small-block uses a 22" wide radiator, 18" high (core size?). I think slant six engines were the same overall size. Some slant six radiators had a smaller core, but the same overall dimensions (w/ sheet metal spacer). I think that was only in the 1970's when they changed to a 26" wide radiator. Anyway, anybody would prefer having a full-width core.
All slant six engines (225 cu in and less, until 1981?) had the lower tube on the passenger's side, all 1.5"D.
Small-block V-8 (273, 318, 340, 360) with the early cast-iron water pump (to 1971?) had the lower tube on the driver's side, all 1.5"D (mine is, and matches the water pump inlet size).
Later small-block V-8's (1972?) have the aluminum water pump. The lower tube is 1.75"D on the passenger's side (matches water pump inlet).
Upper tubes appear to vary, perhaps dependant on AC. I think all are 1.5"D, for slant six, small block, and BB big block (383, 400). Don't know about RB big-block (413, 440). However, the location seems to vary. Most original radiators I have seen in photos have the inlet on the driver's side. However, in my 1965 Newport 383 and 1965 Dart 273, that location would interfere with the after-market AC compressor (York or Tecumseh). I don't know about the Chrysler compressor since I think it was mounted more toward the center, at least by ~1971. My original radiators had the upper tube on the passenger's side of the fill neck, some more toward the edge than others.
Some people switch to the later aluminum water pump in small-blocks (I did), so the year of the car doesn't always matter. That mainly sets the core width. Even with the correct width, some people have found after-market aluminum radiators to be slightly too tall. I even have a copper 22"W one (from B-body?) that is too tall for my Dart (but not Newport). Even though the later Magnum engines are termed "small-block" (5.2L and 5.9L 80's & 90's trucks) and are essentially the same engine, the radiators are probably much different, though many people put those engines in older A bodies.
Once you produce the radiator, the main thing is to fully detail all important dimensions and to have a correct photo. I am amazed how many radiator sellers can't do that, so it is a crap-shoot what you get. Classic car owners are smart enough to figure out if it will fit if you give them the info they need.
