It's best to remove the bumpers. Like Chuck Leaper says, "Why would you install only the missing mounts?" The bushings are probably the original bushings and hardware, which makes them about 30 years old..... Buy a new bushing kit including the GNX bushings and new hardware from Dennis Kirban. Spray the bolts that you can with a lubricant, and the ones that you can't, well then hope they don't break. I did mine and none of them broke. Should be okay. Car will feel much tighter and drive much nicer when you're done. If you have a lift it's pretty easy, without a lift it's a little tougher, but still doable.