My understanding the 64-68 will all fit.
But when I needed a frame for my 66 vert, I went to see a friend of mine who's been collecting & selling 65-68 parts for 20 yrs. He gave me an education on the frame tops.
For 67-68 Ford made a minor change to the bow that goes over front passengers. To increase head height, they formed a step into that rail. This gives front seats a little more top height.
Noting when I looked at my 67 frame, yes it is there, that step. Then I picked up a 66 frame from him, no step.
I too understand that 64.5 through 68 will all fit. However, there are subtle difference between the years including cable tensioners, dowel pins, top bows, and the split glass rear window which I think wasn't originally available until 1967. Ford changed the hinge design for 1968 from a cast to stamped design.