AFAIK
I don't think the trunk floor dimensions of the E9 coupe changed, nor do I think the floor dimensions of the E3 (2500 through 3.3) changed.
I have a coupe which came with the storage "box" at the right side of the trunk, oddly there is no cutout in the carpet for it.
But which is it, coupe or sedan? I dunno.
Perhaps a look-see with realoem?