Difference In Year Of273 Intake/heads

hi, it's 64-65 has a smaller bolt ( 5/16) and at a steeper angle.
the 66 up has 3/8 bolts at different angle. same as a 340 , 318.
performance, is equal. the 920 head is the one of choice though. they all fit.
64-65 manifold should be used with same heads. bolt holes can be redrilled and tapped.