I found a head chart in the Larry Shepard "How to hot rod small block Mopar engines" book. All 273 heads up to and including 1967 had closed chambers in the 57-64.5cc range. 68-69 and 318 heads to 71 had a open chamber in the 60-67.5cc range. And the list goes on and on. Those heads have the 1.78/1.50 valve sizes.
