Slant 6 blocks and tranny matches

the crankshaft register -- is the issue-- look at the one on your 68 if it is the later large style than your tranny is good to go..

I would use the tranny that you know is good. the picture posted is the difference in the register diameter-- i do not remember the dimension-- this is just for a visual reference. the 2 flex plates are from slant 6-- one early one later
Lawrence