Kits are available from Global West and Hotchkis, but if you're good at junkyard scrounging and maybe a little fabrication, you can do it yourself. The only thing non-GM is the upper control arm.
Below are links to other guys that have done the conversion one way or another, and a few with rear disc brakes as well: - The basic tall spindle swap - check out "Whoa Boy" and "Steerage" - Copies of the Hot Rod articles from '87 and '90 on big brakes/tall spindles - Look under the Super Wagon project. Big brakes and rear discs, too! - 13" 'Vette brakes and rear discs, sweet! - 13" brakes, tall spindles, also some info on fast-ratio steering boxes - 12" 'Vette brakes and rear discs. - Home-made tubular A-arms for the tall spindle conversion - Spartan Rod Works' alternative to Global West or Hotchkis control arms

Rear discs:

Improved steering: - rackand pinion steering for early F-bodies. No A-bodies, yet, but... - Fast-ratio steering box info - GM steering box info spreadsheet

Front coil-overs: - QA1 - Carrera

General A-body info and forums:
Real Olds Power!
Team Chevelle

Pro-touring - Handling for older cars - check out the forums, especially the CBB(Cheap Big Brakes) Forum.