Don't use parts from a donor car. Buy a kit with everything you need new. I'd highly recommend 58Delivery - a well known member here who sells disc conversions. His prices are as good as anyone's or better and he has been doing them for a long time. I converted mine before knowing what I do now and although it is acceptable, there are better ways to do it now.
When you talk to him (or anyone) make sure you discuss what wheels you will be using. Both diameter and backspacing or offset as well.
I will be buying from him the next time I put discs on one of mine for the street. As for my '61 Bel Air, I plan on going with a Wilwood set-up to save about 30 pounds sometime down the road. It's pretty much just for drag use. I'd recommend more stock parts for a street car.