I just installed disc brakes to the front of my 67 camaro and now when I start to come to a stop the rear lock up on me. What could be the problem?

What all did you install besides the disc brakes? Master cylinder? Combination valve?

check the following. 1st make sure that you put the shorter of the two shoes on the front(e.g facing the front toward the front bumper)
second make sure your wheel cylinders are not sticking. Last piece just make sure your not overtighting the preload

the kit came with a combination valve, master cylinder and brake booster

did you get the adjustable valve? if you did adjust it in i think in directions it says to increase or decrease. you want to decrease pressure to back.

nope I did not get one of those.