I got a $200 copper radiator from Autozone a few years ago. It was the first Dart one they listed, maybe 1971. It fit the opening fine, but I did have to drill 4 holes in the bracket to match the factory holes. It has a 1.5" D lower tube (passenger side), whereas the small block alum w.p. is 1.75", so I put a rubber bushing adapter on the radiator (might just cut some 1.5" hose). Makes me suspect the radiator was for a slant w/ AC, though the tube isn't angled like slant radiators usually are.
I later got a new aluminum Be Cool radiator off ebay for only $80. If might have just fit the Dart had I cut the frame rail lips off (would fit a 67+ bay). I put it in my 65 Newport instead. I also got a used B-body radiator, and it was too tall to shut the hood, as you found. Many choices out there, so don't jump into anything. I feel stupid having paid $200, since the radiator sat over a year anyway while I worked other issues.
I went to the alum w.p. because of the Autozone radiator, plus it was 1/2 the price. That made me change my p.s. pump to a Federal and play around with pulleys, but finally made things fit.
