The body did change slightly between the 89 and 90 700 series, the rear
panel went from 3 small holes where each vertical pair of bulbs went
through to one larger hole for the whole bulb cluster for the newer type
taillights.  The old taillights bolt directly on to the new cars, the
old cars need the 2 bridges between the 3 small holes removed before the
new lights will fit on.

240s (and 260s) always had one big hole for the taillights, but despite
their similar appearance, 240 and 740 taillights are different enough
that they don't fit the other car.  Of course any part can be made to
fit any car, all it takes is a little skill and effort.  I believe that
the 740 taillight are too tall to fit between the lower edge of the
trunk lid and the bottom of the recess where the lights are mounted.