They are some of the most desirable stock heads for the 460. In the late 60's, early 70's the 460 engine with these heads were putting out over 300 horse power in their stock configuration. If you google them you'll get a ton of hits, some with accurate info, some not. But you will get the gist of it.
Actually I found a link back here where Paul has already added some comments that are really helpfull.
http://www.clubhotrod.com/forums/showthread.php?t=35239