If you were located in Chicagoland I would have been able to help you out.

Switching the internals is not something I think you want to do. You have to set the mesh of the gears which is a precise alignment, requiring an assortment of shims it get it right.

The other problem is that the 3.07 you have now has a 185mm ring gear, the one you are trying to use has a 210mm ring gear. I don't believe it will physically fit in the case.

If I were you, I would sell the diff you have now on eBay (surely someone needs a post -85 diff) and buy the correct one. http://car-part.com/ will help you find one. There are 6 differentials for a 1982 380SE available in the NorthWest, priced from $300-$400.

Otherwise, keep watching eBay. I bought the one that I have in my car for $75, shipped.