I have a 63 and 68 C20 That I'm going to convert to disc. The 63 will be the first in hopefully a few weeks. IIRC if you stay with disc from a 72 or earlier it's a bolt for bolt change. If you get a later disc front end up to 86 there are a few bolt holes off by 1/8 to a 1/4 inch. Here's a thread from Chevytalk.org that has pictures, diagrams, a 128 post, should answer a lot of questions;
http://www.chevytalk.org/fusionbb/showtopic.php?tid/187532/Larry
