We are getting to the point with our SLs, where "wheeler-dealers" put cars together and try to pass them off as original and ask inflated prices. In fairness to all those who try to preserve them and keep them as original as possible - I like to point all of you to "Shvegel's" restauration project, which I think is plain awesome but may seem obsessive to some as far as the details are concerned - I think it matters if the car is original. There is a variety of engines that can be used, albeit not being correct, and I don't think you can brush this off as unimportant.
Yes, there are no exactly matching numbers, because production and assembly was never organized or controlled that way. Still, there is a way to associate correct engine, transmission and chassis numbers for a given car. And why do we have a list of "correct" cylinder heads in the Technical Manual (although it is not complete..)
If you have a "late" 280SL and it carries an early engine before the changes to cooling, it takes away from the value in my book.
If you are in the business of dealing with these cars and push some 20 cars per year through your shop, you may want to downplay the importance of "matching" components