'87 300D: more leg room, more head room.
"87 300D & '86 190D 2.5: Same block, different flywheel and clutch IIRC. The auto is different for the 2.5 and 3.0, different power and torque ratings, different shift points and stall-speed, but
AFAIK it will bolt up and run fine. You'll need to figure out the Bowden cable linkage, vacuum-modulator plumbing, other things.