i dont know i didnt do that i whet 2.8-2.8-305-355. but if i had to guess i would say if you got one from a late 80'* car you would only need the pcm. if i remember right the 88 had 2.8 and 89 started 3.1? if you got a 89 90 seems like they woudnt change much. could be wrong. after that they started getting into vats and stuff not to sure how that would be either.
i wouldnt think that you would have to switch anything unless you are talking about putting the 91 chip in your pcm. which may be all you have to do. i know if you wanted to change fuel or ign settings back then you would have to have your chip flashed.
thats where it starts to be a pain. and thats why i got a carbed 305/700 for like 500 bucks. just get a 70'* sbc hei and mod the 700 slightly, different rad. very easy swap. and you dont have to worry about pcm or sensors going bad any more