What I have read on the German E34 forum: the single rear seat of the E32 fits, but it is difficult to close the doors, unless you remove some of the upholstery stuff on the edges where the door panel touches. The E32 seats are wider in the rear.
Then the E32 has 2 different kind of headrests:
older version it is mounted onto the rear seat back, while in newer versions the plate with the headrest system is mounted onto the steel between passenger room and trunk.
So first of all get the VIN of that 735 you want to remove the parts, use realoem.com and compare the parts and see what is installed.
Wire looms might not be there, so get that also out of the 735, the more the better. You will be surpised how many wire looms are inside a 750 for example, have a look here when I disassembled my parts 750iL
http://www.bimmerboard.com/forums/posts/270025/