Yes, 67 shifter goes thru the floor in a different location. Requires a floor hump and rubber boot that is difficult to find.
I modified the 2 speed shifter in my blue '66 with great results.
First, you need to modify it to provide the extra travel for the 3 speed trans. Solution is to drill a hole in the thru-the-floor lever just below the original, and move the upper shift rod down to the new hole.
Second, the detents need to be modified for the new gear positions. I even added a detent plate for when the button is pushed DOWN for the 1-2 shift.
Thirdly, the linkage below the floor needs to be bent/ground down to provide as mush clearance for the ST400 trans as possible.
No pics on presently on file, but I could do so if there is interest.
Now-a-days, I just recommend buying the cable conversion kit from Shiftworks.
Their Cutlass kit looks like it may work on our Buicks:
http://www.shiftworks.com/cutlas.htm#cutlass66