If your state has emissions testing requirements, you want to look into the requirements before you start. If you are in California, you definitely have to keep your later model sensors, etc. because as I understand it, Calif now makes you conform to the emission standards of the body or the block, whichever is newer.

84 may be a B23. In any case, you will want to use only the long block (block and head) from the donor car and keep your original manifold, wiring, sensors etc., from the 89.

I converted an 88 740 from auto to overdrive. It was a lot of work, but well worth it.

It would be a whole lot easier if you were going in the other direction, from a stick to an auto. The real pain in the butt here is the pedal assembly and swapping it from the stick to the auto car. Everything else is pretty much there, you will need to swap in the old overdrive harness, no big deal, and for the most part past that it is pretty much a direct drop in so long as you have the original donor car to get all of the small pieces like the driveshaft, shifter, relay, etc.

Mark