I know this doesn't answer your question, but personally I feel the 240 seats are more comfortable than those in the 740. If it was me, I'd rebuild the seats you have or find good ones from a boneyard.
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don hodgdon '89 744ti, '81 242t, '71 D-35

It would be a VERY tight fit. The 740 seat is a lot wider- but if you remove the side cover with the seatbelt in it, you might get it to work. A bit of reshaping of the seat track mounts might be needed. They're similar in mounting styles.
I think you'd be happier trying to get a matching 240 passenger seat and rebuild it into your driver's seat. That way you can take care of the sagging seat problem, and have a nice comfy seat beside, and it'll fit without any issues. The seat bottom is identical, you just switch the tracks, and flipflop the control knob. The lumbar knob could be left on the new seatback or you could replace the new seat's back upholstery with your own so the knob fits on the correct (left) side.
My 86 had a driver's seat with intact upholstery, but a broken back frame and trashed foam. I rebuilt an 87 passenger seat into my driver's seat and it's very comfortable. And the seat heaters work!
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Rob Bareiss, New London CT ::: '86 244DL- 215K, 87 244DL- 230K, 88 744GLE- 198K, 91 244 180K

Hello,

I assume you're replacing cause your 240 seats are worn out. The center console would also represent a problem as well. If you have to make new mounting points you might as well get some nice racing seats like some Corbeau's or Recaro's to make it worth your while. Otherwise I would just hold out for a good set of 200 seats. You could rebuild with new foam if your covers are still good, IPD sells the kits.

Happy Bricking!!!

Richard

87 245 DL 351,000 km
82 242 GLT 121,000 km

http://members.cardomain.com/richink