Heres a few places from the links section on this site.

http://www.novak-adapt.com/catalog/sm420_parts.htm

http://www.hermtheoverdriveguy.com/id3.htm

If you feel up to it, I dont think its a real complicated overhaul. I pulled mine apart in a few hours time and the only tools I needed were a handful of sockets, hammer, gear puller, punches, snap ring pliers and a punch. Youll feel pretty good about doing it yourself if you feel up to it!

Heres a helpful write up listed in the parts section and if you have a service manual that pretty much walks through most of the stuff to tear it apart.

http://members.cox.net/cglabe1/SM420/sm420.html

Jeff

My 1953 Chevrolet
1947.1 Gallery
1972 C-10 1/2 Ton & 1972 C-30 1 Ton