No doubt I will miss a detail which someone else will point out, but IME, this is the A65 head selection:
62 - 65 Different valves, 5/16" rear bolts to retain the head by intake rockers. The 5/16" bolts are longer than the later 3/8" bolts and have a boss cast into the head for the longer bolts. IME, the dual carb version of this head is SWEET on the street..
66 - Bigger valves and ports, 5/16" rear mounting bolts
67 - 70 - 3/8" rear bolts, same valves/springs as 66. I have run into a few odd 70's that have 5/16" rocker cover bolts and rockers covers to match
That rocker cover will fit a 71 on head.
71 - 72 - 5/16" rocker cover studs with a slightly different pattern. Rocker cover has 2 threaded holes in top to allow head stead to bolt on.
"Balancing" a head - Mainly making sure the intake port capacities and combustion chamber capacities are matched. It allows similar intake charges and similar compression ratio and combustion side to side. I have never heard any one head was better than any other in that regards. Somewhat luck of the draw depending on tolerances. Some say the 65 and back heads actually had the best intake ports.