things to look for on your old engine issues.
1. pancake batter colored engine oil. if you pop the valve cover and its packed full like it was sprayed in.
causes is anti freeze mixed in oil.
caused by intake gaskets or in you case those blocks had bad castings. they would leak coolant from the lifter valley walls right under the head bolts about 3" down. I have replaced hundreds of them.
2nd and more likely your issue. head bolts
the last bolt on the back of the right head have a bad tendencey of snapping or stretching. if it does so then it leaks coolant to the rear cylinder(right next to the dowel pin hole). and then it slowly takes on the same issues as the "pancake battered" oil. however. it will get the fuel smell going in the oil and the coolant will smell corrosive. its because its building up hydro-carbons in the radiator.