A "core" is bad, like a used car, it's a trade-in to get a better price on new.
As far as I know a t-5 is a t-5. The only thing that changes is the bellhousing (which is seperate) from car to car. But you might want a world-class trans which you can find in an 86-94 (to get bellhousing too) mustang, and 95-03 (I think) for the trans itself since they stopped making the 302 after 94 (until recently, heh).