http://c10forum.com/forum/
Lots of info on that site about 60-66, 67-72, and 73-87's. I haven't built one but from what I remember (could be wrong) reading on many of the build threads is they need 1.5-2" to lay frame and about 3" to lay rocker (with 2x4 inside the stock frame and the bottom 1-2" cut off)