I'm sure they are interchangeable. I don't think that there are many differences between the two motors themselves, if there are any at all. If you mean you are putting an 86 motor in an 89 Mustang, granted they are both n/a 2.3's, you should be fine.
when you say longblock, that generally referse to the intake, shortblock, head and everything.
The 86 has a carb and carb intake. You could use the 86 shortblock and head.