I agree with mjGord entirely. I will add to this a bit.
When upgrading or altering any braking system, it is critical you do it correctly. Correctly in this case MAY mean spending a bit more money and time, yet the headaches getting it ironed out are considerably less.
Many of the companies offering disc brake kits will also provide the correct part numbers for many of the additional parts required (ie: when adding discs to the front, you need this proportioning valve and this master cylinder) and this in itself is priceless. There are some pretty smart people out there who have done all the testing and figuring everything out, then were gracious enough to share that knowledge and experience - take advantage of it.
As I live in Canada, one of the considerations I made when I chose a disc brake kit for my '86 was availability of replacement parts. I would have LOVED a complete Wilwood setup, however with reliability in shipping being quite erratic lately, I chose a kit based on Cadillac parts.
I can not stress enough to follow the manufacturer's directions and never be afraid to call their support line if required - they spent money putting people there for questions.