I have an 88 TII and I was gonna buy a newer 13bt for it.
My question is what is the difference between the 13b from 86-88 and the 13b from 89-91 if any ?
And will the 89-91 bolt into the 88 frame/body?

 

depending on how you want to go about the "upgrade", the things you'd have to worry about are:
1- manual omp and electrical omp
2- front housing cover
3- turbo oil line to front cover housing
4- entire electrical wiring
5- new ecu

 

should bolt right in, but the ECU is different, n332 vs n333 ( us spec anyway ) as long as all the accessories are with the series V your getting you should be ok...