The body plate doesn't have the full VIN unfortunately. And the hardtop is stamped with the body number not the chassis number. But it could still be useful, at least to check whether it matches the other body components. The hardtop number is under the horizontal member below the rear window. You'll need to remove the hardtop and set it upside down to see the number.
TheEngineer's suggestion of using the engine and transmission numbers to obtain the chassis number must be possible. Apparently, there are also unique numbers stamped into each rear axle, left front axle, and steering box. Of course, as you've noted, your engine and transmission are not matched, so at least one and possibly both are non-original. Some of the other stamped parts could have been replaced over the years (does the car have rear drums or discs?), but it's less likely that all would have been replaced. Cross-matching the numbers should be simple for Mercedes but, as Igor and George have said, perhaps not.
You haven't told us whether substituting 113042 into your current VIN (i.e 043049-xx-xxxxxx becomes 113042-xx-xxxxxx) gives a valid number. I'm sure you already explored this, didn't it help?