I have an 86 325is and the tank looks identical to my 91 318i, even has the little hose going from side to side. However, the pump is different, with a different power connector, the filter mount has an aux pump too, and the plumbing is a little different. I think it'd fit, if you swapped the pump and some new lines. Some models had a different tank with a transfer pump too, I'm not sure which ones.
Did you check RealOEM? The pics there can really be helpful, especially year-to-year.