You might want to get the whole car and sell back to them what's left. You'll need more than the block; things like the KE controller and wiring. The 560 came with KE-Jetronic while the 380 came with K-Jetronic with lambda (or something like that). The fuel distributors won't be interchangable. You might be able to convert the 560 to K-Jet with lambda, but that's more than I'd care to try.

Sixto
91 300SE
81 300SD