I'm wandering into the Big bolt section, couldn't resist when I saw the thread on the sm420 since I have one in my GMC 3/4 ton.
Check out this site if you haven't already. Several pictures and commentary of sm420 overhaul and 9 page overhaul manual download available under the first picture.
http://members.cox.net/cglabe1/SM420/sm420.html Good luck on your project. I'm sure there are others here who have actually worked on them and will be glad to help.
