What parts do you need to add rear disks?

Backing plates to hold the calipers on the rear axels, the calipers w/ mounting brackets, the 2 e-brake cables, brake lines going to the rear, and 2 brake hoses to connect to the calipers. You'll probably also need pads and rotors also after all that.

and unless there is a decent proportioning valve already in there, which there wont be, you will need a proportioning valve from a car with rear discs.

and unless there is a decent proportioning valve already in there, which there wont be, you will need a proportioning valve from a car with rear discs. Not necessarily.. the V6's with 2-wheel disc and rear drums had the same proportioning valve as the other Y87 packaged V6's and V8's that came with 4-wheel disc's. I know several people who have swapped the rear drums to discs and left the proportioning valve alone and have had no problems at all with the proportioning of the 2.