The front part of the transmission is just like a normal Chev 3 speed. The back part is a planetary unit with .70 overdrive built by Borg Warner. This is a pretty good over view of the OD
http://www.hydratech.com/pctc/Public_Documents/bwwiring.pdf I would not put an OD in front of 3.42 gears. They are a lot more driveable with 3.90 or 4.11. I have OD in all my Y-blocks.. have for almost 50 years. I have an OD that I am going to put in in a GMC, but it will be with 3.90. I bid on an OD on eBay last week. You may be the one that outbid me at the last 5 seconds, but there will be more.
