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:

http://home.earthlink.net/~cjcarri/brkswap/brkswap.htm - The basic tall spindle swap
http://www.geocities.com/skullmobile/lazylemans.htm - check out "Whoa Boy" and "Steerage"
http://dreinecke.home.att.net/Chevelle/Html/Brakes.html
http://members.tripod.com/CutlassFreak/discs.htm - Copies of the Hot Rod articles from '87 and '90 on big brakes/tall spindles
http://www.einstyn.com/ - Look under the Super Wagon project. Big brakes and rear discs, too!
http://www.geocities.com/diels12000/ - 13" 'Vette brakes and rear discs, sweet!
http://groups.msn.com/70MonteCarlo/my70monte.msnw - 13" brakes, tall spindles, also some info on fast-ratio steering boxes
http://www.buffnet.net/~airrj/My%20Chevelle.html - 12" 'Vette brakes and rear discs.
http://landry-family.com/arms.html - Home-made tubular A-arms for the tall spindle conversion
http://www.westcentralauto.com/chevelle.htm - Spartan Rod Works' alternative to Global West or Hotchkis control arms

Rear discs:
http://www.buickperformance.com/reardisc.htm
http://www.citlink.net/~bowtie/rearbrake.htm

Improved steering:
http://www.speeddirect.com/ - rackand pinion steering for early F-bodies. No A-bodies, yet, but...
http://www.citlink.net/~bowtie/steering.htm - Fast-ratio steering box info
http://www.chevelles.com/techref/shea_3.html - GM steering box info spreadsheet

Front coil-overs:
http://www.qa1.net/shocks/procoilsystems.html - QA1
http://www.carrerashocks.com/GM%20Coilovers.htm - Carrera

General A-body info and forums:
Real Olds Power!
442.com
Team Chevelle
Chevelles.net

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