Not quite sure, maybe this will help?
"Pre '68 F engines used siamesed exhaust ports and a deep "bathtub" style combustion chamber. They're pretty much on their own so far as interchanging anything.
From then until '72 the F engine used a flattop piston and a closed combustion chamber.
'73 and '74 F engines used an open combustion chamber and a very small dome on the piston."