I bought the rearend a few years ago and did not look too close at it. I am missing the lower part where the shocks hook into. Not sure what to call that part. Will the one off my mono leaf 8.2 work? I am putting multi leafs on the car.

That would be the lower shock/spring plate. Your original mono-leaf plates will work, but will move the shock mount a little lower than "normal" since the spring perches for the multi-leaf rear are a bit deeper. Some folks have had issues with the shocks hitting the inside of the wheels (when running wide 15" wheels).

Assuming you have a '67 (based on your sig), then you'll have the non-staggered rear shocks (both shocks on the same side of the axle). In this case, even the "stock" multi-leaf plates wouldn't have worked since they're all set up for staggered shocks. Best bet is to go with the DSE "stock" type shock plates for non-staggered rear shocks. 040302L and 040302R:
http://www.detroitspeed.com/productpages/indproduct/rearsuspensionprod/lower_shock_plates_prod_test.htm