I'd be willing to bet that it's just plain old american steel. I doubt it'd be difficult at all to narrow it if that's what you want to do. If it's chromed buff off the chrome with a flap wheel, cut out a section in the center and weld her back up. Make sure you get good penetration on your weld and I'm sure it will be fine. Bumpers are typically pretty heavy steel so it should be easy.
Keep in mind if you cut out a section in the center and bring them together, the mounting holes will be brought in as well, so you might need to do some changes to the mounting brackets our drill out new holes in the bumper.