I'm not an SL person, so I don't know what years the 450SL covers. If it is the same years as the 123 series, AND you are an experienced engine swapper, I don't think this would be too bad. You would probably have to get hoses made at the local hydraulic hose shop for P/S and A/C, but I think it is very likely that using the correct engine mount castings, you could bolt in the engine and transmission. Since all the SL's of that era were V8's, you need to measure to make sure that you would have the engine compartment length. I would think that if you measured from the center of the vertical motor mount bolt to the rear radiator edge, you would be able to tell.

With this measurement keep in mind that there are at least two different motor mount castings for the 617. They differ from the 126 to the 123 chassis. There also may be a variant for the 115 and 116 chassis that may place the engine properly fore and aft. You will also most likely have to fabricate a driveshaft of the correct length, but maybe not.

Good luck and keep us updated.

Have a great day,