I've seen w140 seats in a w126 SEC convertible before, not sure how hard that was, but it looked great. I had the opportunity to sit in some mint w140 seats in my local junk yard before it closed down (the last good one near me *sob*) and I was shocked by how incredibly comfortable they were.

As for the w123, you can fit W126 electric seats in them physically, its just tricky finding a spot for the controls. This would give you electric\heated seats

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold