Prestone that leaks behind the engine ?

I had the same thing happen to a 1971 small block car I had years ago. The car spent its life in California before making its way to NC in 2003. Not a spot of rust anywhere.......except the freeze plugs, because they never put antifreeze in it in CA.

I took the motor out instead of the transmission. That way, I could get to every freeze plug in it and easily remove them. I pressure washed all the water jackets out and installed new brass freeze plugs. I had the radiator flushed out and they rodded the cores out too. No more problems and the car even ran cooler. It's not a hard job....just time consuming. Good luck to you!