is is possible to put 91 firebird headlights in my 89 firebird formula. Like the entire assembly

Yes. You'll need to swap the nose from a 91-92 also.

Lon

and is that all? just the bumper? or is there more

The 91-92 headlight assemblies use a smaller headlight with a different plug so you will have to get 91-92 headlight pigtails. Also, you'll need 91-92 turn signals with pigtails. We have everything you need available on our site if you're interested in converting over to the 91-92 front end.

you would be able to do it if you swapped the nose. If you just try to change the assembly and motor they won't fit correctly. also im no electrician but the wiring may not match up correctly because the body changed.