Either subframe can be narrowed up to fit the track width but usually by the time you get everything jigged up, cut, and put back together you've wrapped enough time up in them that a MII conversion on the stock frame would be more cost effective.

I will say that I have probably seen more of the front subframe grafts onto an old car done wrong then I have seen done right!!!! Double and triple checking all the measurements before, during, and after the graft is just as important as having things correct on an MII!!!