If you go with 273 heads make sure they are the later style with the 3/8 intake bolts. I know 64 & 65 used smaller 5/16 bolts at a different angle. I think 66 and newer used the 3/8th bolts at the standard angle.
Yes, you want 66-67. There may have been differences there also, but I know that the 2bbl, 4bbl and Commando engine in 67 all used the SAME EXACT head, and that's what I had on mine. Unfortunately the rest of mine was just a 2bbl not a Commando :P
