AOD is a 4 speed transmission, the original C4 is a 3 speed transmission.
You would not have enough selector positions.
Incorrect. The AOD has the same number of shift detents as a C4.
C4
RND21
AOD
RN(OD)D1
Yes the AOD has one more gear, but you don't get a good way to hold 2nd gear. For forward gearing you have the normal Overdrive setting that gets you all 4 gears automaticly. Drive which acts like any old 3-speed automatic in drive, gets you the first 3 gears normally but just blocks Overdrive. And "1" which forces a downshift into 1st gear and holds that gear no matter what, unless you select it at too fast of a speed at which point you get 2nd gear until you slow enough that it drops into 1st and stays there.
The AOD also has a very tall overdrive. 2/3s (.667) overdrive. You need to plan your axle gearing correctly in order to be able to use the overdrive. Remember that this is a Bronco, not a Mustang. Lots more drag. Expect to need roughly 2000 RPM at 60 MPH to cruise the highway speeds. Use any of the online gear ratio calculators and plug in your numbers (including tire size) and see how far off your current gearing will be.
The AOD is also an old archaic transmission. Research the 4R70W. You get a normal 2nd gear (OD cancel is done by a switch or button), you get a proper selectable lockup convertor instead of the bastard design of the AOD. Selectable convertor lockup in normal driving is like getting another gear. The 4R70W is so much better of a transmission.