One important fact I learned from Bob, was that the AMC heads have loose keepers.  The two halves meet each other to touch, thus grabbing the valve stem, being loose, they travel up and down until they break through the grooves and the valve free to move deeper inside and then operated by the piston to have it pushed up.  
So, the first thing I recomment to anyone I meet is to have the keepers ground at their sides until they don't touch each other.  
Bob was very happy with the AMC heads and tossed the old ones for their metal fatigue.  In his experience, it was not worth chancing all the troubles by putting the old ones back.
He liked the better material the AMC heads are made of.  
So, the keepers must be fixed first.  
Zoltan