Carmen,
I had a '70 red/red w/black stripe cowl ind. M22 LS5 3.31 posi w/ BS from Atlanta. My former car's BS is posted over at Chevelle Stuff (
http://www.chevellestuff.com/index.htm ). Good Chevelle reference, btw. From what I have gathered it is not too uncommon to see the LS5 motor coupled to an M22 when a 4 speed was desired. Some folks think this was the only 4 speed coupled with the LS5, but have heard of documented 20s and 21s. It is unusual to see an Arlington built Chevelle with a build sheet. Routine of line workers was not to put BSs in car. Not many around from my experience.
Before you waste too much time ask for detailed photos of eng. stamp pad and partial vin on tranny and undercarriage shots. This car will require some elbow grease as I'll bet the underside is pretty grotey.
Good Luck!
Sam I