I just had this issue with some parts I had bought. I pulled a T5 bellhousing off a 1993 2.3L
stang and it had a 1991 part number on it, meaning a modification was made to the bellhousing starting in the 1991 model year. I bought it anyway and bolted it to the back of a 1986 2.3L and it fit perfectly. It appears the modification done in 1991 was the addition of 2 bottom bolt holes so they could bolt the bellhousing not only to the block, but to the aluminum oil pan that was also introduced that year. All other bolt holes appear undisturbed. So, after my little experiments and checks, I think it would be fairly safe to say that all T5 2.3L bellhousings from 1987-1993 are interchangable.