To use the drop spindles (which are based on '73+ A body spindles) you will need to switch to the '73+ upper control arm with the larger ball joint, or use these adapters on your current UCAs that have a small ball joint:
Mopar A-Body Upper Ball Joint Tapered Adapter
Do a search in the suspension and brake section for details on swapping in the later disc brakes, it will have all the info you'll need to make the switch (which applies to your drop spindles).
Here's a link to one of the suppliers:
Classic Performance CP30100 Classic Performance Drop Spindles | Summit Racing