66-69 had the normal intake bolt size and angle. All had forged crank and just the 64-67 had the mechanical valve train. 68 and 69 went to hydraulic and not adjustable.
I though there was a number of 66' 273s that had the earlier bolt angle-ed heads from 65' on them. I could be wrong (and often am) but I thought I got that information here at some point..
