It will fit perfectly, but there is a difference in operation between 66 and 67. The 66 will be the "green dot" transmission. In 66 Ford made the valve body so when you pull the shifter back from neutral the car starts out in second gear, not first. The idea was that it would be the safest way to accelerate in snow, slick roads etc for inexperienced drivers. To start in first and shift through second and third you had to pull the shift lever back two positions from neutral. On 66 shift indicators this position was shown as a big green dot, thus the nickname. The first spot back from neutral has a smaller dot. Strange to be sure, but true.
It's very common for new 66 owners to post asking about their car not having thrid or first beause they notice they are only getting one shift instead of two. So if you use the 66 you'll have to get used to pulling the lever back an extra spot to what would be L2 on almost every other transmission in the world. If you still have the original 67 you could swap out the valve body and it would act just like the 67.
Another problem with the '66 valve body is if you shift to "LOW" when in third, it will downshift to 2nd gear. Then when the car slows to about 22 MPH, it will downshift to first whether you want it to or not.
When I rebuilt the C-4 on my '66, I had a '67 valve body installed. Now it shifts like your original '67. I installed a '67 stop plate on the shifter so when you pull back from neutral, it stops at "3". It then does a 1-2-3 shift sequence. No more dreaded "Green Dot" shift pattern.