Here's the basic info on the SM420.
http://www.novak-adapt.com/knowledge/sm420.htm

This is a very durable tranny, and is the preferred tranny for the rock crawlers, due to it's low first gear. Unless there are major problems with yours, consider rebuilding yours. Parts are available and the job is not that hard. The factory shop manual has some good instructions. http://chevy.tocmp.com


Fred
52 3600
69 C-10