well-the ignition, exhaust and fuel delivery systems are different.

you HAVE to use the dist, ign ecu and harnesses from the 84. the 89 dist won't turn on the 84 fuel pump relay.

also, the entire fuel system, except the gas tank-front to rear. different pressure than the 89. this includes all the wiring ad sensors on the engine, too.

along with the entire exhaust, its a bigger diameter.

all this assumes the 84s harness is usable and not flaky. there's little things like ac and power steering. the 89 stuff might bolt to the 84 block, not sure. when you're done, the 89 has to look just like the 84 does now under the hood.

want a turbo-fix the 84 and drive it. good luck, chuck