Yes, you would need the Chevy/Olds grille. I would leave the Buick grille in place because it is unique to the car.Is the Caprice and OCC grille shorter (up and down) than a roadmaster?
I was looking for an OCC front bumper cover like you installed so I could get some airflow to the intercooler and not have it look like a hack job, but comparing the two it looks like the Buick bumper has a recessed area between the headlights whereas the Caprice/OCC is more/less straight across. Also the chrome/black trim differences.
I'm guessing that if I went with an OCC bumper, I'd need an OCC or Caprice grille instead of a Roadmaster? I wonder how that would look?
I'm sure you have the skills to come up with something. I know a guy that made a custom bumper that was half Buick, half Mercury Marauder.