Check
http://forums.s-series.org/. The guys there are pretty knowledgable.
As long as the trucks are the same length (both short box or extended cab....), and you switch the engine and tranny you should be fine. The 84 tranny will probably not bolt up... I imagine the 84 has either the 2.5L iron duke or the 2.8L. You would have to use the wiring harness and computer from the 89 as well. Also, you may want to check the gears in these trucks to make sure you aren't "winding-out" the 4.3L to turn the gears in the 84's rear. The RPO code for gears in usually the G codes in the glove box (i.e. GU6=3.42)
This will probably take quite a few weekends and it will probably be best to take off the whole front clip and fenders. Also, be sure to well label everything.
If you run into any trouble with the 89 4.3L or tranny wiring or anything, let me know... I have an 89' 4.3L and a 91' 4.3L I can take pictures of to send you.