You change change 85 parts to 84 engine (may need 85 TBI and other parts so throttle cable works etc.) but...
Is a true 84 engine that often have one to several issues even if seems in "good shape" now.
84 and early cars may had engine replace under warranty/recall and that engine likely Isn't
84 long block but 85-86 long block.
See my Cave, Do You Recall?
& Duke Quick Ref
What's wrong w/ 85 engine?
Some problems are easier to fix then replace the engine.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave