71 Dart Brake Conversion Kit???
Some if not most of us have just found a 73 or newer A-body with disk front brakes and swaped them out. Its easy bolt on. You will need the upper control arm and lower a-arm. You may find the lits in the for sale section.
There are a few places that sell a kit but cost is up there.
there are many treads that talk about it. I will find one and post it here for you to see. I did not document my swap.