I just got a rear disc drive posi unit for my car and unfortunatley the person had cut the brake lines and the brake cables. My current camaro has rear drums. The 2 e-brake cables can they be bought over the counter at any parts store and will it be a direct fit?

You have to adjuust them in at the "T".

You're going to need to change your master cylinder too. There is a specific one for 4 wheel disks.

MC is the same, prop valve is diff

MC is the same, prop valve is diff
Trust me, the master cylinders are NOT the same for drum vs 4 wheel disk. The proportioning valve won't need to be changed, it'll work just fine.