I am posting this as info the the Ford public at large. It is esoteric, but I could not find a simple photo or thread showing the differences in Valve Body design for the C6, relative to the detent for maintaining shifter position inside the transmission, which occurred in the early '70s.
Now I am new to working on these transmissions, so take any of this as you find it.
In my conversion, I wanted to use parts from a '68 C6 in a '73 C6 case, as they had already been rebuilt and the old case was a no-go. The detent system changed, and the news was good for me, I think I can put the older VB in the new case, and here is why:
On the left is the newer valve body. Note the RED circled portion has a long smooth groove. This is for the Case Mounted Detent, seen in the photo below, which is present on the later transmission. In the photo below, the green circle roller rides in the notch on the end of the round shaft and actuates the parking pawl when in ‘Park’.
On the right is the older valve body. Note the GREEN circled portion has grooves in it. There is a spring mechanism inside holding the position at each groove. It does not need the case detent system, and the case detent will just provide additional holding power. It must still activate the parking pawl, which was the same in the old case, but the linkage shown below has no ratchet on it and no leaf spring roller screwed into the case.
Note the ratchet and roller outlined in red. This case mounted detent is NOT present in the old '68 transmission case.
I definitely think you could not put the new VB in the old case, as there would be NO detent system.
If anyone can tell me this won’t work, please do so. However, it looks like I am good to go with the transplant!