Steam at the overflow hose below the radiator cap?

I'm picking the brains of the people who know this better than I. I have the 1971 Dart with the 225 Slant 6. Recently (in the middle of the 100 degree Texas heat), it started spewing steam out the overflow hose coming from the radiator. makes me think the radiator most likely was an aftermarket/replacement. Didn't think the 225s had an overflow and I am afraid it's losing coolant/water over the course of driving it during the week. I loaded it up with almost a gallon of water the other day, but won't go to coolant/antifreeze till I know for sure it's not leaking somewhere I can't see. No water in the oil and the engine is still running fine, just the steam especially when stopped after running. Any ideas? Thanks in advance!