W126 Coupe seats on Sedan
Taken as my very own personal opinion: I sincerely dislike the way Recaro seats look on M-Bs. The Recaros look nice on Sport cars, but they just plainly do not match the M-B interior design theme.
Although, for your W126 sedan, there is a great option you can toy with:
Mercedes-Benz offered in most markets a special seating option that could be ordered for all long-wheel base sedans (300SEL, 380/420SEL, 500SEL and 560SEL). It was called "Coupe Seating for the W126 sedan" or "Individual 4-place seating".
The option mainly included two W126 SEC's front seats (with the folding rear-entry access deleted) and two individual seats for the rear (very similar in shape to the front seats) divided by a wooden topped storage console.
Both the front AND rear seats were fully electrically adjustable, and all four seats also came with heating elements (as a separate option on some markets).
The option called to be upholstered in "Alcantara", a very expensive cloth material exclusive only to the SEC and to the SEL with the 4-place individual seating.
I think if you can find a couple of SEC seats at a junkyard and replace your original ones, and then have the whole interior upholstered in new leather or a similar material to alcantara it will look much better.
This particular option called for an additonal US$12,000.00 to be added to the base price of your long-wheel base sedan. Hence, you do NOT see many of the sedans fitted with expensive alternative. See the photo below: