I had a similar problem with my cousins 68 charger. We rebuilt the 440 and it would run for about 1/2 an hour then start spewing antifreeze and overheat.We thought it was head gaskets, but it turned out to be the radiator was about 3/4 plugged with crud.The antifreeze was clean and looking inside from the rad cap,the tubes looked good with no signs off any build up of corrosion.We had it recored and it fixed the problem completely.Just a thought if your rad is old with a fresh engine.....