By "Early model case", do you mean a 66 only case? I understand that the 66 valve body is an oddball, but if replacing the actual valve body itself, wondering what the issue is? If you mean the style of spring loaded shifter detent design, years ago, when I installed a Ford Motorsport/Art Carr full manual, reverse pattern valve body, I ran into a situation, where there was an issue with the valve body setup for the leaf spring roller detent on the inside shifter lever, but my case was the other style, which had a detent spring in the valve body. The other style VB had the provision for the detent spring, so I just swapped the spring and pin from my stock valve body, into the new VB. Worked just fine.