trans swap

[BASBR71]

3 speed manual to AOD swap , what complications might I encounter in a 71 model

 

[76YETI]

I would look at the Bronco Tech portion of the home page....I quickly found this:

The cross member will need to be modified and relocated to make up for the 1.5" longer AOD. The stock transfer case shifter also requires some modification and a larger hole needs to be cut in the floor. An aftermarket transmission shifter or modification to your stock automatic shifter will be required. Transfer case to floor may be tight if you don't have a body lift installed and the front drive shaft may be very close to the transmission pan depending on your lift. A smaller diameter front drive shaft or transmission pan modification may be required.

 

[Broncobowsher]

Axle gearing being completely incorrectly matched to run the overdrive, regear the axles to use the overdrive.
We are assuming you are starting with a V8, if not there will be an engine swap as well.
AOD isn't that great of a choice in transmissions. 4R70W and 6R80 are much, MUCH better choices.