It's definitely a '66 carb. However, if it came off a Mustang, it has the incorrect throttle linkage. The Bronco throttle linkage was unique from what I understand and carbs of the same vintage from other vehicles with the different linkage won't work correctly.
This is all what I have heard/learned from reading threads here on the board over the years. I have never dealt with a '66 6-cyl carb personally although I have seen them and yours is the correct vintage.