If ALL you want to do is add disc brakes up front leaving everything else "pretty much" stock then one option is the Scarebird conversion. You really should plan on putting in a dual res master cylinder and proportioning valve.  This kit uses Stanza rotors with Cavalier calipers, reportedly works fine, but some stock steel wheels won't work and will require either a wheel change or another kit which will work with deeper rims which some cars have. If you already have aftermarket wheels then they most likely will fit the cheaper kit. There is some machining involved and do NOT press out the old studs for the new ones or you'll be buying new hubs.

With six cylinder cars there are other options. A couple others on this board have made their own awesome, ingenious conversions. Search this subject on this board and the old one.

Now, if you want to upgrade to 5 lug V8 equipment...