Trying to decipher this...
The C6 and E4OD are not good choices for an early Bronco. They are simply HUGE! They don't fit very well.
What can bolt to these transmissions? Ignoring that they are a bad choice, the NP205 is about the only junkyard option that will bolt on without adapters.
The Dana 20, without aftermarket adapters, only has 2 factory options. The C4 and 3-speed manual. The Dana 20 is a unicorn that only works with factory Bronco stuff. Even so you still need the Bronco version of the Dana 20 and not other versions. Or an adapter.
Lets back up and try and find you the right transmission. You have a '66 Bronco. Guessing you are swapping the I6 for a V8?
What are your plans for this Bronco? How will you drive it? What do you want it to do? What size tires? Axle gearing?