Check the water pump, Look closely at the weep hole. Easier to find leak if you have a pressure tester. I have seen some not leak until warmed up and under pressure. Careful when using tester to not exceed psi on rad cap and release pressure on tester before removing from radiator.
A co-worker told me about a site carsonline.com . I was surfing around there and this interesting little Dart caught my eye. A lot of drool material on that sight.
http://www.cars-on-line.com/26745.html
Hot Rod Magazine addressed this problem in the June issue. Here is a link the article. http://www.hotrod.com/techarticles/engine/flat_tappet_cam_tech/
Bill